WARRING-STATES JAPAN, 1467-1600 Nicholas D. Anderson

THE POLITICS OF WARRING-STATES JAPAN, 1467-1600

Nicholas D. Anderson Department of Political Science, Yale University [email protected]

ABSTRACT: This article introduces “The Politics of Warring-States Japan, 1467-1600,” a new collection of data sets covering political and military relations between warlords within Japan during its warring-states period from 1467 to 1600. Drawn from the most authoritative chronology of the politics of the Japanese archipelago during the warring-states period, data were collected on seven topics of interest: battles between Japan’s warlords (n=2,889); territorial conquest within the Japanese archipelago (n=1,224); alliances among Japan’s warlords (n=576); gift-giving by Japanese warlords (n=448); surrender by Japanese warlords (n=112); natural disasters occurring within the Japanese archipelago (n=656); and various attributes of Japan’s premodern provinces. Focusing primarily on the battle data, this article introduces the sources of the data, describes the collection procedures and coding rules, presents basic descriptive statistics of key variables of interest, and applies the battle data to an important question in the international relations literature: whether conflict “contagiously” diffuses across time and space. The data introduced here should be of interests to scholars of international relations, civil conflict, Early Modern East Asia, and Japanese history, among many others.

24 March 2021 7,735 Words (5,041 without notes)

This is a work in progress, so I gladly welcome questions and/or comments.

Nicholas Anderson is a visiting scholar at the Institute for Security and Conflict Studies at the Elliott School of International Affairs at The George Washington University, and a Ph.D. candidate in the Department of Political Science at Yale University. He would like to express his gratitude to Jonathon Baron, Mary Elizabeth Berry, Ja Ian Chong, Thomas Conlan, Fabian Drixler, Jonathan Markowitz, Steven Miller, Nuno P. Monteiro, Sebastian Peel, Christopher Price, Philip Streich, Monica Duffy Toft, Remco Zwetsloot, and especially Frances Rosenbluth, for their insightful and constructive comments on earlier drafts of this article. Haruko Nakamura provided crucial guidance and support at Yale’s Sterling Memorial Library. This research would not have been possible without the consistently outstanding research assistance of Makiko Shirado. The project also benefited greatly from presentations at Yale University, the Massachusetts Institute of Technology, the University of Southern California, and annual conferences of the International Studies Association (ISA) in 2017 and the American Political Science Association (APSA) in 2017, 2018, & 2019. This research was generously supported by the Japan Foundation’s Center for Global Partnership and Yale University’s Center for East Asian Studies.

Key words: Japan; East Asia; Early-Modern History; Quantitative Data In recent years, research in international relations and international security studies has begun to look backwards, to examine what lessons emerge from relations among earlier forms of social organization. Some, for instance, have examined the warring states of ancient China, or regional relations in early modern East Asia.1 Others have explored the Christian crusades, and conflict and cooperation in medieval Europe.2 Others still have looked to early modern Europe, to see what lessons the relations between actors of this period hold for today.3 And some have even explored the relations between hunter-gatherer bands, the most primitive form of human social organization.4

However, one place and period that seems to have escaped much scholarly scrutiny within international relations is Japan of the late-medieval period.5 Existing research within political science primarily focuses on the Edo period (1603-1868) that followed,6 is more firmly rooted in comparative politics,7 or both.8 This is unfortunate, since the “Warring-States Period” (Sengoku Jidai,

1 Victoria Tin-bor Hui, War and State Formation in Ancient China and Early Modern Europe (New York: Cambridge University Press, 2005); David C. Kang, East Asia Before the West: Five Centuries of Trade and Tribute (New York, NY: Columbia University Press, 2010); Ji-Young Lee, China’s Hegemony: Four Hundred Years of East Asian Dominance (New York: Columbia University Press, 2016). 2 Michael C. Horowitz, “Long Time Going: Religion and the Duration of Crusading,” International Security, Vol. 34, No. 9 (Fall 2009), pp. 162-193. On the medieval period, see: Markus Fischer, “Feudal Europe, 800-1300: Communal Discourse and Conflictual Practices,” International Organization, Vol. 46, No. 2 (Spring 1992), pp. 427-466; John G. Ruggie, “Territoriality and Beyond: Problematizing Modernity in International Relations,” International Organization, Vol. 47, No. 1 (Winter 1993), pp. 139-174. 3 Hendrik Spruyt, The Sovereign State and its Competitors: An Analysis of Systems Change (Princeton: Princeton University Press, 1994); Daniel H. Nexon, The Struggle for Power in Early Modern Europe: Religious Conflict, Dynastic Empires, and International Change (Princeton, NJ: Princeton University Press, 2009); John M. Owen, The Clash of Ideas in World Politics: Transnational Networks, States, and Regime Change, 1510-2010 (Princeton, NJ: Princeton University Press, 2011). 4 Jack Donnelly, “The Elements of Structures of International Systems,” International Organization, Vol. 66, No. 4 (October 2012), pp. 609-643. 5 Important exceptions are: Philip A. Streich, “The Failure of the Balance of Power: Warring States Japan, 1467-1590,” (Ph.D. Dissertation, Rutgers University, 2010); Philip Streich, “The Balance of Power in Japan’s Warring States Period,” Asia Pacific World, Vol. 3, No. 2 (Autumn 2012), pp. 17-36; Ji-Young Lee, “Hegemonic Authority and Domestic Legitimation: Japan and Korea under Chinese Hegemonic Order in Early Modern East Asia,” Security Studies, Vol. 25, No. 2 (June 2016), pp. 320-52; Lee, China’s Hegemony. 6 Erik Ringmar, “Performing International Systems: Two East Asian Alternatives to the Westphalian Order,” International Organization, Vol. 66, No. 1 (2012), pp. 1-25; Abbey Steele, Christopher Paik, and Seiki Tanaka, “Constraining the Samurai: Rebellion and Taxation in Early Modern Japan,” International Studies Quarterly, Vol. 61, No. 2 (June 2017), pp. 352-370. 7 John A. Ferejohn and Frances McCall Rosenbluth, eds., War and State Building in Medieval Japan (Stanford: Stanford University Press, 2010). 8 Mark Ravina, “State-Building and Political Economy in Early-Modern Japan,” The Journal of Asian Studies, Vol. 54, No. 4 (November 1995), pp. 997-1022. 1

1467-1600) was one of great social and political fragmentation, followed by relentless warfare, political consolidation, and eventual unification. In this period, the Japanese archipelago descended into a state of anarchy, not unlike the international system itself. And the units that emerged, their various forms of interaction, and the eventual unification of the country by the late 16th century, may hold important lessons for our understanding of the central problem of international relations— conflict and cooperation in the absence of centralized authority.

In this article, I present new data that will help scholars of international relations and international security studies grapple some fundamental questions in new and important ways. The

“Politics of Warring-States Japan, 1467-1600” data cover political relations between warlords within the Japanese archipelago during the 15th and 16th centuries. Using the most comprehensive, authoritative chronology of Japanese history during the warring-states period, I compiled data on a variety of political phenomena, including battles (n=2,889), territorial expansion (n=1,224), alliances

(n=576), gift-giving (n=448); surrender (n=112), natural disasters (n=656), and various attributes of

Japan’s premodern provinces. In this article, I focus primarily on the battle data, and only briefly on the other data.

No quantitative data of this breadth and detail currently exists. Much of the existing research on

Japan’s warring-states period is within the field of history, consisting of either narrowly-focused but richly-detailed studies,9 or broadly-focused but more sparsely-detailed studies.10 The existing

9 See, for example: Peter Judd Arnesen, The Medieval Japanese Daimyo: The Ouchi Family’s Rule of Suo and Nagato (New Haven, CT: Yale University Press, 1979); Mary Elizabeth Berry, Hideyoshi (Cambridge: Harvard University Press, 1982); Carol Richmond Tsang, War and Faith: Ikko Ikki in Late Muromachi Japan (Cambridge: Harvard University Press, 2007); Peter D. Shapinsky, Lords of the Sea: Pirates, Violence, and Commerce in Late Medieval Japan. (Ann Arbor: University of Michigan Press, 2014). 10 For broad historical overviews, see: George Sansom, A History of Japan, 1334-1615 (Stanford, CA: Stanford University Press, 1961); John Whitney Hall, Nagahara Keiji, and Kozo Yamamura, eds., Japan Before Tokugawa: Political Consolidation and Economic Growth, 1500-1650 (Princeton: Princeton University Press, 1981); Kozo Yamamura, ed., The Cambridge History of Japan: Volume 3, Medieval Japan (New York: Cambridge University Press, 1990); John Whitney Hall, ed., The Cambridge History of Japan: Volume 4, Early Modern Japan (New York: Cambridge University Press, 1991); William Farris, Japan to 1600: A Social and Economic History (Honolulu: University of Hawaii Press, 2009); Karl F. Friday, ed., Japan Emerging: Premodern History to 1850 (Philadelphia, PA: Westview Press, 2012). 2 quantitative data that focuses on premodern East Asia does not cover Japan, focusing solely on

China, Korea, and Vietnam.11 And the only other quantitative data collection covering this period in

Japan of which I am aware is Peter Brecke’s “Conflict Catalogue,” and it includes just 48 conflicts for the warring-states years.12 Thus, the “Politics of Warring-States Japan, 1467-1600” data will be a useful additional tool with which to examine this important period of world history.

In outlining the data, I first provide a very brief historical overview of warring-states Japan, to help contextualize the description of the data that follows. Second, I introduce the source of the data, and describe the coding rules and procedures followed in the construction of the battle data.

Third, I present basic descriptive statistics regarding the most important variables in the battle data.

Fourth and finally, I present an application of the battle data to an important question in international relations research—whether conflict “contagiously” diffuses over space and time— before concluding.

Historical Overview

Japan’s warring-states period ran from the outbreak of the Ōnin War in 1467 to the end of the

Battle of Sekigahara in 1600, and was marked by near-constant social upheaval and violent conflict throughout the Japanese archipelago. A succession dispute within the ruling Ashikaga Clan sparked the Ōnin War in early 1467, leading to conflict that would eventually engulf the entire country.13 The

11 David C. Kang, Meredith Shaw, and Ronan Tse-min Fu, “Measuring War in Early Modern East Asia, 1368-1841: Introducing Chinese and Korean Language Sources,” International Studies Quarterly, Vol. 60, No. 4 (2016), pp. 766-777; Mark Dincecco and Yuhua Wang, “Violent Conflict and Political Development Over the Long Run: China Versus Europe,” Annual Review of Political Science, Vol. 21 (May 2018), pp. 341-358; David C. Kang, Dat X. Nguyen, Ronan Tse- min Fu, and Meredith Shaw, “War, Rebellion, and Intervention under Hierarchy: Vietnam-China Relations, 1365-1841,” Journal of Conflict Resolution, Vol. 63, No. 4 (April 2019), pp. 896-922; Tackseung Jun and Rajiv Sethi, “Extreme Weather Events and Military Conflict Over Seven Centuries in Ancient Korea,” PNAS, Vol. 118, No. 12 (23 March 2021), pp. 1- 6. 12 Peter Brecke, “Violent Conflicts 1400 A.D. to the Present in Different Regions of the World,” Paper Prepared for the 1999 Meeting of the Peace Science Society (1999). Data available at: http://www.cgeh.nl/data. 13 Mary Elizabeth Berry, The Culture of Civil War in Kyoto (Berkeley and Los Angeles: University of California Press, 1994). 3 power and authority that had been held by the shōgun (military ruler) since the 12th century and the emperor (ritual sovereign) since at least the 6th century diminished to the point of virtual nonexistence.14 Powerful warlords, known as daimyō, emerged in the upheaval, consolidating power within their home provinces and regions.15 Some powerful regional warlords turned their attention outwards, upon neighboring territories and toward the capital, Kyoto. In the process, numerous warlords and their clans rose and fell, as they fought to maintain their territory and vied for military supremacy. Three would eventually rise above the rest—Oda Nobunaga (1534-1582), Toyotomi

Hideyoshi (1537-1598), and Tokugawa Ieyasu (1543-1616)—and would become known as the “three unifiers” of Japan.16 Acting both in concert and in succession through the latter third of the 16th century, these three pacified the daimyō, unified the archipelago, and ushered in the early modern era of Japanese history.17

Battle Data: Collection, Coding Rules, and Procedures

“The Politics of Warring-States Japan, 1467-1600” data was almost entirely collected from Shin

Kokushi Dai-Nenpyo (hereafter, “Shin Kokushi”), the most comprehensive and authoritative chronology of Japanese history available at the time of collection.18 This award-winning, Japanese- language chronology spans a century and a half, covers political, economic, social, and cultural

14 Asao Naohiro and Marius Jansen, “Shogun and Tennō,” in Hall, Nagahara, and Yamamura, eds., Japan Before Tokugawa, pp. 248-270. 15 David Eason, “Warriors, Warlords, and Domains,” in Friday, ed., Japan Emerging, pp. 233-243. 16 On the unification of Japan in this period, see: Berry, Hideyoshi, ch. 4; Asao Naohiro and Bernard Susser, “The Sixteenth Century Unification,” in Hall, ed., The Cambridge History of Japan: Vol. 4, pp. 40-95. 17 Marius B. Jansen, The Making of Modern Japan (Cambridge, MA: Harvard University Press, 2000), p. 21. 18 Eigou Hioki, ed., Shin Kokushi Dai-Nenpyo [新國史大年表], Vol. IV, 1456-1600: Konran no Sengoku Jidai, Nobunaga, Hideyoshi, Ieyasu (Tokyo: Kokusho Kankoukai, 2009). In a handful of cases—mostly surrounding the Siege of Odawara (btl_id: 2785) and the Battle of Sekigahara (btl_id: 2877)—Shin Kokushi was supplemented with other sources. Those sources were: Yabe Kentarō, “The siege of Odawara by Toyotomi Hideyoshi and the Seiga-Nari Daimyo” [“秀吉の小田原出兵と「清華成」大名”], Kokugakuin Daigaku Kiyo, Vol. 49 (February 2011), pp. 131-149; Sanbō Honbu, Nihon no Senshi: Sekigahara no Eki [日本の戦史関ヶ原の役], Vol. 6 (Tokyo: Tokuma-shoten, 1994), pp. 214-230; JapanKnowledge Lib (Tokyo: NetAdvance, Inc., 2020), Available at: https://japanknowledge.com. 4 events, and totals over one thousand pages of text.19 Every event listed in the chronology is cited to a primary or a secondary source. The Japanese historian Naramoto Tatsuya refers to the chronology as a “majestic achievement,” providing “quality information for each event” in Japanese history. He is echoed by fellow historian Irokawa Daikichi, who states that the “contributions of this great chronology to academics cannot be overestimated.”20

In terms of the battle data, the unit of analysis is the individual battle and the structure of the data is dyadic. For the purpose of this data set, a battle is defined as the organized use of military force by at least one army on at least one other army, or by at least one army on a military target, such as a castle or fortified town. This definition excludes a number non-battle uses of force in this period of

Japanese history, such as robberies, assassinations, duels between individuals or small groups, bloodless surrenders, and the raiding by armies of non-fortified towns. To give the reader a sense of what battle looked like at the time, armed clashes typically consisted of anywhere from a few dozen to a few thousand personnel on either side, some on horseback and some on foot, armed with bows, spears, swords, and as time went by, muskets and horse-drawn cannons. They often fought in and around castles and castle towns, with battles lasting from a few hours to a few months, in the case of sieges.21

A typical example of the type of entry that would be included in the data set as a battle is the

Fourth War of Kawanaka Island in Shinano Province, between the famed warlords Uesugi Kenshin and Takeda Shingen on the tenth day of the ninth month of 1561 (btl_id: 1649). The entry in Shin

Kokushi notes that “Uesugi Masatora’s (Kenshin) army battled with the army of Takeda Shingen at

19 For an alternative, less comprehensive chronology, see: Katō Tomoyasu, et al., eds., Nihon Shi Sōgō Nenpyō [日本史総合年表] (Tokyo: Yoshikawa Kōbunkan, 2005). For a less comprehensive chronology focused only on the warring-states period, see: Futaki Kenichi, ed., Nenpyō Sengoku-shi [年表戦国史] (Tokyo: Shin Jinbutsu Ōraisha, 1978). 20 Catalogue for Shin Kokushi Dai-Nenpyo [新國史大年表], Vol. IV, 1456-1600, Kokushokankokai (2011), Available at: https://www.kokusho.co.jp/np/isbn/9784336048295/. 21 Stephen Turnbull, War in Japan, 1467-1615 (Oxford: Osprey Publishing, 2002), pp. 15-29, 35-66. 5

Shinano Kawanaka-island,”22 a clear case for inclusion in the data set. It is also important to note that the only battles included in the data set are those that took place within the Japanese system, defined geographically as the three islands containing the 68 premodern provinces of Japan—

Honshu, Kyushu, and Shikoku. This means that battles taking place on extra-systemic territories, such as Ezo-chi (now Hokkaido) to the north, China and Korea to the east, and the Ryukyu

Kingdom (now Okinawa Prefecture) to the south, are not included in this data set. Battles were chosen rather than more general wars for both methodological and practical reasons.

Methodologically, counting battles rather than aggregating up to wars gives the data greater granularity, enabling more detailed studies to be conducted. Practically, there were many battles that were one-off skirmishes, that didn’t necessarily fit into larger and broader wars. Furthermore, when war is a relative constant, as it was during this period, much of the time it is very difficult to separate individual battles from more general wars.

For each individual battle, the data set includes information on a number of other important variables. The year, month, and day of the initiation of each battle is included, when that information was available.23 The actors, or participants, in each battle are also included. The criteria for inclusion as an actor engaging in battle is having been the leader of an independent army that participated in a battle. Often these were warlords (daimyō), their retainers, or their generals, but not always, periodically including religious sects, peasant armies, masterless warriors (rōnin), and even the shōgun himself in a few instances. The location of each battle is also included in the data, both the province (kuni) and the more specific location within that province, such as the county (gun), town, castle, or temple. Information is also included regarding which side initiated the battle. The rules

22 “上杉政虎（謙信）、信濃川中島で武田信玄と交戦する。” Eigou, ed., Shin Kokushi, p. 585. 23 If the entry included a year and month but not a day, or a year, but not a month or a day, it was included. If the entry was not certain which year a battle took place in, it was not included. Dates are included in both the premodern Japanese lunisolar calendar and the modern Gregorian calendar. Dates were converted using NengoCalc v4, http://www.yukikurete.de/nengo_calc.htm. See the codebook for more details. 6 for coding a unit as having initiated a battle were straightforwardly whether it was reported as such in Shin Kokushi. A clear example of how an initiation was coded comes from an attack by Hōjō

Ujitsuna on the eleventh day of the seventh month of 1537 (btl_id: 1127). Shin Kokushi clearly notes for this date that “Hōjō Ujitsuna’s army attacked Musashi Kawagoe castle,” a straightforward case of initiation.24

Information was also included on battle outcomes, whether one unit or another saw victory, or whether the outcome was a draw. There were four classes of cases in which a battle was coded as a victory for one side or another. The first is when Shin Kokushi straightforwardly reports a victory for one army or the surrender of another. For example, for the third day of the eighth month of 1574

(btl_id: 2155), the source notes that “Oda Nobunaga’s army attacked on the Ikkō Sect army at Ise

Ō-Torī bastion, and won.”25 If victory wasn’t clearly reported, then a second type of instance coded as a victory was when Shin Kokushi reports that one army flees from another after battle. An example of this is for the twenty-fourth day of the ninth month of 1487 (btl_id: 381), the source writes that

“Hosokawa Masamoto, Takeda Kuninobu, and Togashi Masachika… attacked Rokkaku Takayori at

Ōmi Kannon-ji castle. Rokkaku escaped to Kōga castle.”26 A third set of instances coded as a victory was when Shin Kokushi reports that an army’s leader dies in battle. For instance, for the ninth month of 1495 (btl_id: 486), the source notes that “The assistant governor of Owari domain Oda

Hyōgonosuke battled with fellow-clan-member Oda Toshinobu. Oda Toshinobu and his brother died in the battle.”27 A final class of instances in which a battle was coded as a victory was when Shin

Kokushi reports that the specific territory upon which the battle was waged (ex. castle) changed

24 “北條氏綱、武蔵河越城を攻撃する。” Eigou, ed., Shin Kokushi, p. 453. 25 “織田信長、一向宗徒を伊勢大鳥居に攻めて陥れる。” Eigou, ed., Shin Kokushi, p. 690. 26 “細川政元・武田国信・富樫政親… 近江観音寺城の六角高頼を攻撃。高頼は甲賀城に逃れる。” Eigou, ed., Shin Kokushi, p. 202. 27 “尾張守護代織田兵庫助、一族の織田敏信と戦い、敏信兄弟は戦死する。” Eigou, ed., Shin Kokushi, p. 257.

7 hands in the aftermath of the battle. An example of this is for the fourth day of the second month of

1562 (btl_id: 1666), Shin Kokushi notes that “Matsudaira Motoyasu’s (Tokugawa Ieyasu) army attacked on Mikawa Kaminogō castle, and occupied it…”28 Battles were coded as draws when Shin

Kokushi reports that the battle was a draw or had no decisive winner.29 Finally, information is included as to whether armies fought as coalitions, with more than a single army on one side, on the other, or both—again, based on direct reporting from Shin Kokushi. Table 1, below, summarizes

Table 1: Summary of Battle Data Variables btl_id A unique numerical identifier for each individual battle observation. year The year during which the battle was initiated. month The month during which the battle was initiated. day The day upon which the battle was initiated. actorA A battle participant on one side. actorB A battle participant on the other side. location The specific location of the battle. province The province in which the battle took place. initiatorA Whether actorA initiated the battle (1/0). victoryA Whether actorA was victorious in the battle (1/0). victoryB Whether actorB was victorious in the battle (1/0). draw Whether the battle ended in a draw (1/0). coalitionA Whether side A fought as part of a coalition (1/0). coalitionB Whether side B fought as part of a coalition (1/0). irregular Whether the battle involved an “irregular” actor (1/0). extra_systemic Whether the battle involved an actor from outside the Japanese system (1/0). naval Whether the battle had a naval component (1/0). siege Whether the battle had a siege component (1/0). actor_count A count of the number of actors on each side of the battle. source The source(s) relied upon to code the observation.

28 “松平元康（徳川家康）、三河上郷城を攻撃して、陥れ…” Eigou, ed., Shin Kokushi, p. 589. 29 For example, for 1526/12/15 (btl_id: 804), Shin Kokushi notes “Satomi Sanetaka of Awa battled with the armies of Hojo Ujitsuna’s generals, Ito Syuzen and Ogasawara Genzaemon, at Kamakura. The battle was not settled, and Satomi’s army retreated.” “安房の里見実堯、北條氏綱の将伊東主膳・小笠原源左衛門らと鎌倉に戦う。決着はつかず里見軍は後退する。” Eigou, ed., Shin Kokushi, p. 396.

8 all of the variables included in the battle data.30

Battle Data: Descriptive Statistics

“The Politics of Warring-States Japan, 1467-1600” battle data totals 2,889 individual battles from the beginning of 1467 to the end of 1600.31 The number of battles varies considerably over the course of the years covered in the data set, bottoming out at between 0 and 1 from 1592-1598, and peaking at

84 battles in 1582.32 Figure 1, below, illustrates the trend of battles over time. There is also notable

Figure 1: Battles by Year, 1467-1600 90 80 70 60 50 40 30 20 10 0

variation in the distribution of battles over the months of the year, with the spring months and, more obviously the summer months, standing out as “fighting seasons” in this period.

30 There are a number of other variables in the data set, such as the size of the forces for each side, whether the battle or an attack was ordered by a third-party actor, the number of casualties suffered by either side, whether the battle had a naval component, whether the battle had a siege component, whether the battle had an extra-systemic participant from outside the Japanese system, whether the battle was irregular, pitting two different types of actors against one another, such as a warlord army against a religious sect, or a warlord army against the Shōgun, and whether a coalition member joined (joiner) an ongoing battle. Please see the codebook for more details. 31 For a list of some of the more historically-prominent and well-known battles included in the data set, please see Table A1 in the Appendix. 32 This figure has to be interpreted with some caution, as the pattern seen in Figure 1 is consistent with greater data availability over time, rather than a secular increase in the number of battles. 9

Figure 2: Battles by Month, 1467-1600 350

300

250

200

150

100

0 1月 2月 3月 4月 5月 6月 7月 8月 9月 10月 11月 12月

Figure 2, above, presents the distribution of battles by month in the data set.33 333 individual battles are coded as having taken place in the eighth month of the year, while just 155 took place during each of the first and twelfth months. Note also the marked decline during the sixth month of the year, very likely due to annual monsoon rains.

Figure 3: Battles by Province, 1467-1600 200 180 160 140 120 100 80 60 40 20 0

33 The figure presents months based on the traditional lunisolar calendar. It does not include so-called “leap months.” Please see the codebook for more details. 10

There is also important geographic variation in the number of battles in the data set, with some provinces, such as the capital province, Yamashiro, seeing 195 individual battles, and others, such as island provinces like Sado and Shima, seeing few to none. Figure 3, above, displays the five provinces with the greatest and the five provinces with the fewest battle observations in the data.34

Figure 4, below, similarly presents the number of battles each medieval province experienced between 1467 and 1600, though in cartographic form.

Figure 4: Battles by Province (Map), 1467-1600

Finally, there is a lot of interesting variation in terms of battle participation in the data set. Of course, to be included in the battle data a commander would have to have partaken in at least one battle, and the data includes 2,223 unique battle participants overall. The vast majority of these—

34 Please see Table A2, in the Appendix (below), for a full listing of the number of battles in each of Japan’s 68 medieval provinces. 11 approximately 2,100 or 95 percent—participate in ten or fewer battles.35 However, a number of prominent actors engaged in considerably more than this. Figure 5, below, lists the ten actors with

Figure 5: Battles by Actor, 1467-1600 0 20 40 60 80 100 120

Oda Nobunaga (織田信長) Mōri Motonari (毛利元就) Takeda Shingen (武田信玄) Uesugi Kenshin (上杉謙信) Tokugawa Ieyasu (徳川家康) Toyotomi Hideyoshi (豊臣秀吉) Matsunaga Hisahide (松永久秀) Amago Haruhisa (尼子晴久) Kobayakawa Takakage (小早川隆景) Kikkawa Motoharu (吉川元春)

Battles Initiations the greatest number of battle observations in the data.36 And it shows what we would expect—the names of famed warlord clans, such as Mōri, Oda, Takeda, Tokugawa, Toyotomi, and Uesugi. The figure also indicates the number of times these individuals’ armies initiated battles, providing something of an aggressiveness ratio, with armies such as those of Oda Nobunaga and Toyotomi

Hideyoshi attacking roughly 80 percent of the time, and armies such as that of Amago Haruhisa, for instance, initiating battle only 30 percent of the time.

The initiation, victory, and coalition data also reveal some interesting patterns. Overall, in 2,150 of the total 2,889 battles (74 percent), the data includes information on which side initiated the battle. Similarly, in 2,105 battles (73 percent), we have information on which side was victorious in battle. Putting these two together shows that, among battles where we know who initiated and who

35 And a full 1,264 participate in just a single battle. 36 Note that only the leaders of independent armies are included as battle participants, so individual participants’ aggregate battle numbers are generally be conservative, as many likely took part in battles as subordinates, prior to leading their own, independent armies. 12 won, roughly 82 percent of the time the initiating side was victorious, and just 18 percent of the time was the non-initiating side victorious. Coalitions were involved on one or both sides of a battle in

1,147 battles in the data set (40 percent). In 197 cases (9 percent) both sides fought as a coalition, in

739 cases (33 percent) only one side fought as a coalition,37 and in 1,281 instances both sides fought independently (53 percent).38 Table 2, below, reports summary statistics for each of the key variables in the battle data.

Table 2: Summary Statistics of Battle Data Variables Variable Range Observations Percentage missing Mean Year 1467-1600 2,889 0 1543 Month 1-12* 2,800 3.1 6.6 Day 1-30** 2,421 16.2 — ActorA — 2,858 1.1 — ActorB — 2,226 22.9 — Location — 2,489 13.8 — Province — 2,765 4.3 — InitiatorA 1/0 2,150 25.6 — Victory 1/0 2,105 27.1 — Draw 1/0 2,105 27.1 0.02 CoalitionA 1/0 2,845 1.5 0.31 CoalitionB 1/0 2,119 23.2 0.21 Irregular 1/0 2,889 0 0.11 Extra_systemic 1/0 2,889 0 0.002 Naval 1/0 2,889 0 0.01 Siege 1/0 2,889 0 0.07 Actor_countA 1-56 2,801 3.0 1.55 Actor_countB 1-26 2,183 24.4 1.30 * Some months are displayed as ranges (e.g., 10~11), some are displayed as seasons (e.g., “summer”), and some are “leap months” (e.g., [10]). See the codebook for full details. ** Some days are displayed as ranges (e.g., 10~11). See the codebook for full details.

37 These two figures are based on battles for which we have coalitional information for both sides, totaling 2,217 observations. 38 This figure is based on battles for which we have coalitional information for both sides, or we know there was a coalition on at least one side, totaling 2,428 observations. 13

Other Data

As noted above, I collected data on a number of other political phenomena beyond battles during

Japan’s warring-states period. First, I collected dada on territorial expansion, which I define as the coercive (i.e., non-voluntary) transfer of territory from one actor to another. The territorial expansion data totals 1,224 individual observations, and each observation includes a number of important variables, including the date on which it took place, the territory gained, the actor gaining the territory, and the actor losing the territory. Second, I collected data on alliance formation, which

I define as a relatively durable political and military alignment between at least two actors. The alliance formation data totals 576 observations, and each comes with a number of important variables, including the date on which the alliance was formed, the participants on each side, the rival actor against which the alliance was formed, and a series of dichotomous variables indicating any formal binding mechanisms included with the agreement. Third, I collected data on gift-giving, which I define as the voluntary donation of a tangible thing of value from at least one actor to at least one other actor. The gift-giving data totals 448 observations, and each is accompanied by a number of important variables including the date on which the gift was given, the giving and receiving actors, a qualitative description of the gift, and a series of dichotomous variables indicating the broad type of gift that was given, whether it be economic, military, ceremonial, or natural.

Fourth, I collected data on surrender, which I define as the voluntary submission of one actor to another actor in the absence of the direct use of force. The surrender data consists of 112 observations, and each observation includes a number of important variables, including the date on which the surrender took place, the surrendering actor, and the actor who is being surrendered to.

Fifth, I collected data on natural disasters, which I define as a major adverse event resulting from natural processes. The natural disaster data totals 656 observations, and each observation is accompanied by a number of important variables, including the date of the event, the location of the

14 event, and a qualitative description of the event, including severe storms, earthquakes, flooding, famine, droughts, tsunamis, and the like. Sixth and finally, I collected data on Japan’s 68 premodern provinces. The province data includes a number of important variables for each province, including its area (km2), its region, the ruggedness of its terrain, and its longitudinal and latitudinal coordinates.

The key source used and the collection procedures for these other data sets is the same as for the battle data described above. Further information on all of these data sets can be found in their respective codebooks.

To sum up, the breadth and detail of the battle and other data will be of use to researchers in a variety of fields, including Japanese history, military history, international relations, and comparative politics. However, it is important to consider a potential source of weakness of the data sets. This is that almost all of the data are derived from a single source, Shin Kokushi Dai-Nenpyo.39 This means, of course, that the data will only be as accurate as Shin Kokushi, and that any biases in the selection or description of historical events will be directly reflected in the data. The good news, as noted above, is that this is the most detailed and comprehensive chronology of Japan’s warring-states period available, including hundreds of thousands of events across over one thousand pages of text, and with each event cited to a primary or secondary source. While this doesn’t completely rule out event- selection bias or inaccuracy, it does help reduce them to the greatest extent possible within a single source.

Application: The Diffusion of Conflict

Is conflict “contagious”? Does it diffuse over space and time? These questions have been asked, and the diffusion of conflict has been observed, in a variety of geographic settings and in a number of forms of political violence. This section reexamines the diffusion of conflict in the context of

39 For the few exceptions, see fn 18. 15 warring-states Japan, using the battle data presented above. I find that conflict does appear to diffuse spatially and temporally. However, this result only holds between provinces sharing a major road.

These findings serve as a reminder of the importance of logistical influences on the diffusion of conflict. And this is particularly true in the highly-mountainous, logistically and technologically rudimentary setting, of late-medieval Japan.

Diffusion, in the context of political phenomena, occurs when events or policy decisions in a given geographic area are systematically conditioned by prior events or policy decisions made in other geographic areas.40 The diffusion of conflict, therefore, occurs when conflict in a given area is systematically conditioned by prior conflict in other areas. The diffusion of conflict has been observed in a wide variety of geographic settings and in a number of different forms of political violence, including interstate war,41 civil war,42 ethnic conflict,43 insurgency,44 rebellion,45 and terrorism.46 In examining processes of conflict diffusion, research has also found evidence for the

40 This is a paraphrasing of Simmons, Dobbin and Garrett’s definition of “international policy diffusion.” See: Beth A. Simmons, Frank Dobbin, and Geoffrey Garrett, “Introduction: The International Diffusion of Liberalism,” International Organization, Vol. 60, No. 4 (Autumn 2006), p. 787. For a more general, less spatial, definition, see: David Strang, “Adding Social Structure to Diffusion Models: An Event History Framework,” Sociological Methods & Research, Vol. 19, No. 3 (February 1991), p. 325. 41 Benjamin A. Most and Harvey Starr, “Diffusion, Reinforcement, Geopolitics, and the Spread of War,” The American Political Science Review, Vol. 74, No. 4 (December 1980), pp. 932-946; Randolph M. Siverson and Harvey Starr, “Opportunity, Willingness, and the Diffusion of War,” The American Political Science Review, Vol. 84, No. 1 (March 1990), pp. 47-67. 42 Håvard Hegre and Nicholas Sambanis, “Sensitivity Analysis of Empirical Results of Civil War Onset,” Journal of Conflict Resolution, Vol. 50, No. 4 (August 2006), pp. 508-535; Idean Salehyan and Kristian Skrede Gleditsch, “Refugees and the Spread of Civil War,” International Organization, Vol. 60, No. 2 (Spring 2006), pp. 335-366; Kristian Skrede Gleditsch, “Transnational Dimensions of Civil War,” Journal of Peace Research, Vol. 44, No. 3 (2007), pp. 293-309; Kristian Skrede Gleditsch and Nils B. Weidmann, “Richardson in the Information Age: GIS and Spatial Data in International Studies,” Annual Reviews in Political Science, Vol. 15 (2012), pp. 461-481; Nathan Black, “When Have Violent Conflicts Spread? Introducing a Data set of Substate Conflict Contagion,” Journal of Peace Research, Vol. 50, No. 6 (2013), pp. 751-759. 43 Halvard Buhaug and Kristian Skrede Gleditsch, “Contagion or Confusion? Why Conflicts Cluster in Space,” International Studies Quarterly, Vol. 52, No. 2 (June 2008); Lars-Erik Cederman, Luc Girardin, and Kristian Skrede Gleditsch “Ethnonationalist Triads: Assessing the Influence of Kin Groups in Civil Wars,” World Politics, Vol. 61, No. 3 (July 2009), pp. 403-473. 44 Yuri M. Zhukov, “Roads and the Diffusion of Insurgent Violence: The Logistics of Conflict in Russia’s North Caucasus,” Political Geography, Vol. 31, No. 3 (March 2012), pp. 144-156. 45 Nathan Danneman and Emily Hecken Ritter, “Contagious Rebellion and Preemptive Repression,” Journal of Conflict Resolution, Vol. 58, No. 2 (2014), pp. 254-279. 46 Manus I. Midlarsky, Martha Crenshaw, and Fumihiko Yoshida, “Why Violence Spreads: The Contagion of International Terrorism,” International Studies Quarterly, Vol. 24, No. 2 (June 1980), pp. 262-298; Michael C. Horowitz, “Nonstate Actors and the Diffusion of Innovations: The Case of Suicide Terrorism,” International Organization, Vol. 64, 16 mitigating effects of peacekeeping,47 repression,48 state capacity,49 and cross-border intervention.50

While some have found that the diffusion of conflict is less common than is generally supposed,51 there is significant scholarly consensus that it is a general phenomenon.

These findings have been explained a number of ways, with different research pointing to different causal mechanisms. Some have argued that the cross-border refugee flows that accompany violent conflict are the crucial driver of its diffusion to new areas.52 Other research has found that diffusion is more likely when territories share transnational ethnic ties.53 Other candidate mechanisms that have been put forward include the movement of war materials across borders, demonstration effects associated with rebellion and conflict, contested issues that span interstate boundaries, and regional economic decline that results from conflict.54 Despite these differences, what these scholars agree upon is that the outbreak of conflict cannot be adequately understood by focusing solely on factors within a given territory or country.

No. 1 (Winter 2010), pp. 33-64; Robert Braun and Michael Genkin, “Cultural Resonance and the Diffusion of Suicide Bombings: The Role of Collectivism,” Journal of Conflict Resolution, Vol. 58, No. 7 (2014), pp. 1258-1284; Gary Lafree, Min Xie, and Aila M. Matanock, “The Contagious Diffusion of Worldwide Terrorism: Is it Less Common Than We Might Think?” Studies in Conflict & Terrorism, Vol. 41, No. 4 (2018), 261-280. 47 Kyle Beardsley, “Peacekeeping and the Contagion of Armed Conflict,” The Journal of Politics, Vol. 73, No. 4 (October 2011), pp. 1051-1064. 48 Danneman and Ritter, “Contagious Rebellion and Preemptive Repression.” 49 Alex Braithwaite, “Resisting Infection: How State Capacity Conditions Conflict Contagion,” Journal of Peace Research, Vol. 47, No. 3 (2010), pp. 311-319. 50 Jacob D. Kathman, “Civil War Contagion and Neighboring Interventions,” International Studies Quarterly, Vol. 54, No. 4 (December 2010), pp. 989-1012. 51 Black, “When Have Conflict Spread?”; Erika Forsberg, “Diffusion in the Study of Civil Wars: A Cautionary Tale,” International Studies Review, Vol. 16, No. 2 (June 2014), pp. 188-198; Lafree, Xie, and Matanock, “The Contagious Diffusion of Worldwide Terrorism.” 52 Salehyan and Gleditsch, “Refugees and the Spread of Civil War.” 53 Buhaug and Gleditsch, “Contagion or Confusion?” 54 For overviews, see: Danneman and Ritter, “Contagious Rebellion and Preemptive Repression,” p. 255; Forsberg, “Diffusion in the Study of Civil Wars,” pp. 190-192; Kathman, “Civil War Contagion and Neighboring Interventions,” p. 992. 17

Figure 6: Total Number of Provinces that have Experienced Conflict by Year, 1467-1600

Premodern Japan consisted of 68 provinces.55 As noted above, the Ōnin War began in the city of

Kyoto (in Yamashiro Province) in early 1467, and from there, conflict would rapidly engulf much of the archipelago. Figure 6 (above) shows the total number of provinces that have experienced at least one battle by year. As the figure indicates, within just ten years 43 provinces had experienced at least one battle, this number reached 60 by the 1520s, and all but one Japanese province had experienced battle by 1584. Of course, in any given year most of the archipelago did not experience conflict. Figure 7 (below) tracks the percentage of provinces experiencing battle in any given year.

As the figure indicates, roughly 12 percent of provinces experienced battle each year up until 1530, approximately 18 percent from 1530 to 1560, and about 20 percent thereafter, with 1581 standing out as the year of the widest distribution of battle, at 35 percent of provinces.

55 The system was established in 645 CE, and after reforms in 824 CE, it would remain in place until the Meiji Restoration of 1868. See: “Provinces (国),” in JapanKnowledge (2020), https://japanknowledge.com. 18

Figure 7: Percentage of Provinces Experiencing Conflict by Year, 1467-1600

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

So, conflict was clearly widely, though unevenly, distributed in Warring-States Japan, and increasingly so as time went by. In order to examine whether diffusion processes were at work in this spread and increase in conflict, I use the province as the spatial unit and the year as the temporal unit, making the “province-year” the unit of analysis.56 The outcome of interest, or the dependent variable (DV), is a dichotomous variable indicating whether at least one battle took place in a given province in a given year (war). The primary independent variable (IV) is also dichotomous, indicating whether at least one battle took place in a territorially-proximate province in the recent past

(adj_war).57 I operationalize this variable conservatively, considering only provinces that share a land border with the province in question as “territorially-proximate,” and only battles that took place in the same calendar year (though, of course, at an earlier date) or the prior calendar year as “recent.”58

I exclude from the analysis premodern Japan’s five island provinces—Awaji (淡路), Iki (壱岐), Oki (隠

岐), Sado (佐渡), and Tsushima (対馬)—as they have no land borders. Thus, with 63 provinces over the course of 134 years, the data includes a total of 8,442 province-years.

56 The structure of the data is monadic. Others have used dyadic data to examine the diffusion of conflict. See: Black, “When Have Conflicts Spread?”; Forsberg, “Diffusion in the Study of Civil Wars,” pp. 195-196. 57 Therefore, even if multiple proximate provinces experience battle in the recent past, or there are multiple battles within a proximate province, it still only counts as a single observation. 58 Thus, the shortest possible duration between “recent” battles is a single day (ex., 1/1/1467 & 1/2/1467) and the longest possible duration is just shy of two years (ex., 1/1/1467 & 12/30/1468). 19

However, as Yuri Zhukov rightly points out, much of the diffusion of conflict research ignores the significant logistical constraints that the spread of conflict will often face.59 These constraints are bound to be particularly acute in the setting of late-medieval Japan. The issue is not only that, as a volcanic island archipelago, Japan is highly mountainous,60 but that the historical period under examination is associated with extremely rudimentary infrastructure and transportation technology.

Thus, as a second IV, I include another dichotomous variable indicating whether at least one battle took place in the current or prior year in an adjacent province that shares a major road with the province in question (road_war). Premodern Japan had a fairly extensive road network, which was completed over the course of the 7th to 9th centuries.61 Given the logistical challenges of the time and the topography of the archipelago, this would likely have been the primary or sole conduit by which conflict would spread.

The analysis also includes a number of control variables. First, I control for neighbors, a simple count of the number of neighboring provinces a given province has. Second, to account for the possibility that conflict might be correlated with province size, I control for area, the territorial area

(km2) of a given province. And third, to account for the relationship between terrain and conflict propensity, I control for terrain_ruggedness, the average topographic variability of each province using the Terrain Ruggedness Index.62 Table 3 provides summary statistics for the variables.

59 Zhukov, “Roads and the Diffusion of Insurgent Violence.” 60 Japan is in the 77th percentile globally, according to average Terrain-Ruggedness Index measures. See data accompanying Nathan Nunn and Diego Puga, “Ruggedness: The Blessing of Bad Geography in Africa,” The Review of Economics and Statistics, Vol. 94, No. 1 (February 2012), pp. 20-36. 61 For the map used as the source for the road network data, see: Appendix: Figure A1. 62 The terrain ruggedness index is a quantitative measure of topographic variability. It is the mean of the absolute difference between a central cell and its eight surrounding cells. See: Shawn J. Riley, Stephen DeGloria, and Robert Elliot, “A Terrain Ruggedness Index that Quantifies Topographic Heterogeneity,” Intermountain Journal of Science, Vol. 5, No. 1-4 (1999), pp. 23-27; Giuseppe Amatulli, et al., “A Suite of Global, Cross-Scale Topographic Variables for Environmental and Biodiversity Modeling,” Scientific Data, Vol. 5, No. 180040 (2018). 20

Table 3: Summary Statistics Variable Mean SD Min Max war (DV) 0.1598 0.3664 0 1 adj_war 0.5961 0.4907 0 1 road_war 0.4485 0.4974 0 1 neighbors 4.032 1.642 1 10 area 4,531.6 6338.4 343.4 48,432.1 terrain_ruggedness 203.7 81.36 15.3 394.2 Note: there is no missing data on any variable.

Analysis

To examine the relationship between recent conflict in an adjacent province and conflict in a given province, I use a linear probability model. In order to control for potential unobserved confounds across subjects or time periods, I include two-way fixed-effects, for province and year. I also report

“robust” standard errors in order to account for error heteroskedasticity.

Table 4 (below) presents the results. To start, the relationship between conflict in a given province and recent conflict in an adjacent province appears to be weak, if it exists at all. The coefficient on adj_war in model 1 is substantively small and not quite statistically significant by conventional standards, though it is close. However, adding the alternative IV of conflict in an adjacent province connected by a road (road_war) in model 2 produces a much stronger result. The coefficient here indicates a much stronger relationship and it is highly-statistically-significant, whereas the coefficient on the simple adjacency variable (adj_war) is now indistinguishable from zero. The analysis suggests that having at least one battle occur in a neighboring province in a given or prior year is associated with a 3.3 percent increase in the probability of experiencing battle in a given province. Given that the baseline probability of observing battle in any given province-year is approximately 16 percent (see Table 3), this increase is substantial. Thus, the analysis suggests that conflict did diffuse geospatially in premodern Japan, though this only or primarily seems to occur between provinces sharing a major road.

Table 4: Linear Probability Analysis ======Dependent variable: ------war (1) (2) ------adj_war 0.018* -0.007 (0.009) (0.013)

road_war 0.033*** (0.013)

neighbors 0.137*** 0.135*** (0.020) (0.019)

area 0.0001*** 0.0001*** (0.00001) (0.00001)

terrain_ruggedness -0.001*** -0.001*** (0.0004) (0.0004)

------Observations 8,442 8,442 R2 0.136 0.136 ======Note: *p<0.1; **p<0.05; ***p<0.01

Robustness Tests

I conducted a number of robustness tests to see how sensitive these results are to alternative model specifications and the inclusion of alternative controls. The full results are in the Appendix, though I briefly describe the procedures and findings here. First, I reran the analysis using Logistic regression and the results were similar, though the coefficient on road_war is only significant at the 0.1 level with this model.63 Second, I reran the analysis with a lagged-dependent variable (war_lag) rather than two-way fixed effects and the result are similar, though the coefficient on road_war is now substantively larger.64 Third, I reran the analysis, but instead of two-way fixed effects, I controlled for peace_years, a simple count variable of the number of years since a given province last observed battle.65 The results are similar to those with the lagged DV, though the coefficient on adj_war is now highly-statistically-significant in both models. And fourth, I reran the analysis controlling for each of

63 See Appendix: Table A3. 64 See Appendix: Table A4. 65 All provinces start in 1467 at 1, unless they experience conflict that year, in which case they start at 0. See Appendix: Table A5. 22 premodern Japan’s eight regions, one-by-one, to see if any was importantly driving the results.66 The main results are unchanged.67

Conclusion [91]

In this article I have introduced “The Politics of Warring-States Japan, 1467-1600,” new quantitative data covering political relations between warlords on the Japanese archipelago during Japan’s warring-states period. The data should be of interest to scholars of Japanese history, early modern

East Asia, and systemic international relations theory. Furthermore, since no quantitative data of this sort currently exist, the hope is that this project will open up this fascinating period of world history to scholars from a variety of fields, from military history, to international relations, to comparative politics, and beyond.

66 Regions are defined by the ancient Goki Shichidō (five home provinces, seven circuits) system. See: “Goki Shichidō (五畿七道),” in JapanKnowledge (2020), https://japanknowledge.com. 67 See Appendix: Tables A6 & A7. 23

Appendix

Table A1, below, lists a few of the more notable and well-known battles and wars of the Warring-

States period, and their dates and unique IDs in the battle data.

Table A1: Notable Battles & Wars, 1467-1600 Date Battle btl_id 1467/1/18 Outbreak of the Ōnin War 1 1493/[4]/22 The Coup of Meiou 441 1495/9 Hōjō Conquest of Odawara Castle 485 1546/4/20 Night Battle of Kawagoe 1297 1551/8/27 Tainei-ji Incident 1403 1553/8 First Battle of Kawanaka-jima 1451 1555/7/19 Second Battle of Kawanaka-jima 1497 1555/10/1 Battle of Itsuku-shima 1506 1557/8 Third Battle of Kawanaka-jima 1554 1560/5/19 Battle of Okehazama 1607 1561/9/10 Fourth Battle of Kawanaka-jima 1649 1564/8/3 Fifth Battle of Kawanaka-jima 1754 1571/9/12 The Siege of Mount Hiei 2034 1572/12/22 Battle of Mikatagahara 2070 1575/5/21 Battle of Nagashino 2186 1577/10/10 Battle of Shigisan Castle 2268 1580/4/9 Siege of Ishiyama Hongan-ji 2386 1582/6/2 The Honnoh-ji Incident 2489 1583/4/21 Battle of Shizugatake 2558 1584/4/9 Battle of Owari-Nagakute 2603 1585/8/6 Hideyoshi’s Conquest of Shikoku 2667 1587/4/17 Battle of Nejiro-zaka 2728 1590/4/1 Seige of Odawara 2785 1600/9/15 Battle of Sekigahara 2877

Table A2, below, is a list of the number of individual battles observed in each of Japan’s 68 medieval provinces from 1467-1600.

Table A2: Battle Observations by Province, 1467-1600 Yamashiro (山城) 195 Kōzuke (上野) 46 Dewa (出羽) 17 Settsu (摂津) 123 Ōsumi (大隅) 45 Hōki (伯耆) 15 Shinano (信濃) 120 Izumi (和泉) 44 Sanuki (讃岐) 15 Mutsu (陸奥) 116 Owari (尾張) 42 Tango (丹後) 15 Hizen (肥前) 114 Shimotsuke (下野) 40 Noto (能登) 14 Ōmi (近江) 113 Tanba (丹波) 38 Awa2 (阿波) 13 Yamato (大和) 104 Hitachi (常陸) 37 Izu (伊豆) 13 Chikuzen (筑前) 85 Kai (甲斐) 37 Mimasaka (美作) 13 Aki (安芸) 82 Satsuma (薩摩) 35 Inaba (因幡) 12 Echigo (越後) 70 Tosa (土佐) 35 Awa1 (安房) 7 Hyūga (日向) 69 Bingo (備後) 33 Hida (飛騨) 7 Kawachi (河内) 69 Iwami (石見) 30 Kazusa (上総) 7 Musashi (武蔵) 64 Sagami (相模) 30 Iga (伊賀) 6 Mikawa (三河) 61 Kaga (加賀) 30 Tajima (但馬) 6 Higo (肥後) 58 Kii (紀伊) 29 Iki (壱岐) 5 Mino (美濃) 58 Suruga (駿河) 26 Awaji (淡路) 4 Harima (播磨) 55 Shimōsa (下総) 26 Nagato (長門) 4 Buzen (豊前) 53 Chikugo (筑後) 25 Tsushima (対馬) 4 Tōtōmi (遠江) 53 Bizen (備前) 21 Oki (隠岐) 3 Echizen (越前) 51 Bungo (豊後) 21 Wakasa (若狭) 3 Ise (伊勢) 50 Bicchū (備中) 19 Sado (佐渡) 2 Ecchū (越中) 49 Suō (周防) 19 Shima (志摩) 0 Izumo (出雲) 47 Iyo (伊予) 18 Unknown 124

Figure A1, below, is the map used to generate the road network data.

Figure A1: Japan’s Ancient Provinces with Road Network

Source: “五畿七道”, 日本大百科全書（ニッポニカ）, JapanKnowledge (2020), https://japanknowledge.com.

Table A3, below, presents the results of a logistic regression analysis as a robustness test for the main results presented in Table 3, above.

Table A3: Logistic Regression Analysis ======Dependent variable: ------war (1) (2) ------adj_war 0.152* 0.001 (0.088) (0.123)

road_war 0.192* (0.108)

neighbors 1.087*** 1.075*** (0.178) (0.178)

area 0.001*** 0.001*** (0.0002) (0.0002)

terrain_ruggedness -0.013*** -0.013*** (0.003) (0.003)

Constant -5.054*** -5.011*** (1.128) (1.126)

------Observations 8,442 8,442 Log Likelihood -3,091.359 -3,089.666 Akaike Inf. Crit. 6,576.719 6,575.332 ======Note: *p<0.1; **p<0.05; ***p<0.01

Table A4, below, presents a linear probability analysis as a robustness test for the main analysis presented in Table 3, above. It includes a lagged dependent variable rather than two-way (province- year) fixed effects.

Table A4: Linear Probability Analysis with Lagged DV ======Dependent variable: ------war (1) (2) ------adj_war 0.046*** 0.008 (0.008) (0.011)

road_war 0.052*** (0.012)

war_lag 0.308*** 0.304*** (0.014) (0.014)

neighbors 0.013*** 0.013*** (0.003) (0.003)

area 0.00000*** 0.00000*** (0.00000) (0.00000)

terrain_ruggedness -0.0002*** -0.0001*** (0.00005) (0.00005)

Constant 0.051*** 0.046*** (0.011) (0.011)

------Observations 8,379 8,379 R2 0.123 0.125 Adjusted R2 0.122 0.124 Residual Std. Error 0.343 (df = 8373) 0.343 (df = 8372) F Statistic 233.993*** (df = 5; 8373) 198.973*** (df = 6; 8372) ======Note: *p<0.1; **p<0.05; ***p<0.01

Table A5, below, presents a linear probability analysis as a robustness test for the main analysis presented in Table 3, above. It includes a peace_years rather than two-way (province-year) fixed effects.

Table A5: Linear Probability Analysis with Peace_years ======Dependent variable: ------war (1) (2) ------adj_war 0.068*** 0.028*** (0.008) (0.011)

road_war 0.054*** (0.011)

peace_years -0.006*** -0.006*** (0.0002) (0.0002)

neighbors 0.004 0.004 (0.003) (0.003)

area 0.00000*** 0.00000*** (0.00000) (0.00000)

terrain_ruggedness -0.0001** -0.0001* (0.00005) (0.00005)

Constant 0.193*** 0.185*** (0.013) (0.013)

------Observations 8,442 8,442 R2 0.109 0.112 Adjusted R2 0.109 0.111 Residual Std. Error 0.346 (df = 8436) 0.346 (df = 8435) F Statistic 206.910*** (df = 5; 8436) 176.526*** (df = 6; 8435) ======Note: *p<0.1; **p<0.05; ***p<0.01

Tables A6 and A7, below, present linear probability analyses as a robustness tests for the main analysis presented in Table 3, above. Each model includes a control for one of premodern Japan’s eight regions.

Table A6: Linear Probability Analysis with Provinces ======Dependent variable: ------war (1) (2) (3) (4) ------adj_war -0.007 -0.007 -0.007 -0.007 (0.013) (0.013) (0.013) (0.013)

road_war 0.033*** 0.033*** 0.033*** 0.033*** (0.013) (0.013) (0.013) (0.013)

neighbors 0.135*** 0.135*** 0.104*** -0.005 (0.019) (0.019) (0.022) (0.099)

area 0.0001*** 0.0001*** 0.00004 0.00004 (0.00001) (0.00001) (0.00003) (0.00004)

terrain_ruggedness -0.001*** -0.001*** -0.001 -0.002*** (0.0004) (0.0004) (0.001) (0.0003)

reg_tousan -1.357*** (0.193)

reg_toukai 0.057 (0.044)

reg_hokuriku -0.129 (0.099)

reg_kinai 0.266 (0.205)

Constant -0.146 -0.146 0.010 0.499 (0.160) (0.160) (0.144) (0.439)

------Observations 8,442 8,442 8,442 8,442 R2 0.136 0.136 0.136 0.136 Adjusted R2 0.116 0.116 0.116 0.116 Residual Std. Error (df = 8244) 0.345 0.345 0.345 0.345 F Statistic (df = 197; 8244) 6.611*** 6.611*** 6.611*** 6.611*** ======Note: *p<0.1; **p<0.05; ***p<0.01

Table A7: Linear Probability Analysis with Provinces ======Dependent variable: ------war (1) (2) (3) (4) ------adj_war -0.007 -0.007 -0.007 -0.007 (0.013) (0.013) (0.013) (0.013) road_war 0.033*** 0.033*** 0.033*** 0.033*** (0.013) (0.013) (0.013) (0.013) neighbors 0.135*** 0.135*** 0.114*** 0.135*** (0.019) (0.019) (0.018) (0.019) area 0.0001*** 0.0001*** 0.0001*** 0.0001*** (0.00001) (0.00001) (0.00001) (0.00001) terrain_ruggedness -0.001*** -0.001*** -0.002*** -0.001*** (0.0004) (0.0004) (0.0003) (0.0004) reg_nankai -0.039 (0.070) reg_sanin 0.057 (0.046) reg_sanyou -0.074 (0.057) reg_saikai -0.127** (0.062)

Constant -0.146 -0.146 0.017 -0.146 (0.160) (0.160) (0.145) (0.160)