A growing group of investigators is using world history to the benefit of international relations (IR) theory. In a seminal study, Kaufman, Little, and Wohlforth (2007) argue that large spans of non-European history discard the balance of power theory, one of the most important pillars of the realist paradigm. Hui (2005) contrasts balancing behaviors in modern Europe with ancient China, while Yankey-Wayne (2017) shows the relevance of offensive realism in the case of the eighteenth and nineteenth-centuries Gold Coast. Kopalyan (2017) and Wilkinson (2005) summarize the duration of different types of international structures throughout history. Eckstein (2006) applies neorealism to Rome’s rise in the third and second centuries B.C.

Yet, although this research trend has been dynamic for the last two decades, most discussions of international relations theory still overwhelmingly focus on modern and contemporary cases. Quantitative studies exploring history further back than the Battle of Waterloo are rare occurrences. As such, much of human history is ignored, and the universe of data available for testing and refining theories is limited to a short span of time (Blachford 2021). This led many to suggest that most IR theories are irrelevant outside the modern West (Acharya 2014; Acharya and Buzan 2019; Anderl and Witt 2020; Foulon and Meibauer 2020).

This article introduces a new dataset to remedy the lack of historical data that plagues the IR field. In aggregate, the dataset represents 55 centuries worth of data. I code polarity and the relative distribution of power throughout seven distinct international systems. I also count the central wars (wars involving most great powers) that occurred and their duration. These central wars are the yardstick used to compare the different systems. In a nutshell, the data do not support most of the main IR approaches except for classical and offensive realism.

I describe the coding in the next part. In the third part, I explain the historical narrative behind the data. Lastly, I emphasize the main findings and relate them to the predictions of the leading IR approaches. The reader will also find a summary of the core data in the appendix and a supplementary online material offering a more exhaustive and visual rendition of the dataset.

International System

An international system arises when two or more states interact together. It consists of “political units that maintain with each other regular relations and that are all capable of being involved in a general war ” (Aron 2004, 103, emphasis in original) and so is “formed much like a market: it is individualistic in origin, and more or less spontaneously generated as a byproduct of the actions of its constitutive units” (Ruggie 1983, 265). In such a system, “the behavior of each is a necessary factor in the calculations of others” (Bull and Watson 1984, 1). The principal and distinctive organizing principle of an international system is anarchy. For many centuries, several international systems independent from each other coexisted; “All international systems have been an-archical, in the strict sense of the term: they have not been subjected to an archē ” (Aron 1966, 483). After 1860, separate international systems gave way to a single global international system.

Central War

Using central wars for a yardstick is a methodology inspired by Copeland (1996, 9) and Mearsheimer’s (2014, chap. 9) measurement of systemic violence by counting central wars — sometimes also called major wars, general wars, or hegemonic wars (see Colby 2021, 17–9; Gilpin 1988, 600–2; Levy 1985, 364–5). Counting only central wars excludes other types of warfare. Indeed, a certain international structure may produce few central wars but numerous smaller wars, thus making it seem more peaceful than it actually is. Therefore, the present study does not attempt an exhaustive and definitive comparison and focuses only on the likelihood of central war.

A central war, as defined by Levy (1983, 75), is a high-intensity conflict involving at least two third of the great powers in a given system. It is often a conglomerate of bilateral great power wars forming a coherent strategic sequence, such as World War II.¹ I also identify the initiator(s) for each central war.² Full years are used as time units throughout the dataset.

Great Power and Polarity

A state is a political entity that has control over a territory and a population. This entity must be capable of unitary military action. If it does not have effective control over its own foreign policymaking, such as in the case of a colony, then it cannot be considered a state (Fazal 2004, 312). Its internal arrangement matters little. It can be a nation-state, an empire, a kingdom, a city-state, a caliphate, a people’s republic, or anything else; “it does not matter what kind of political units make up the system, as long as it is anarchic and the threat of violence is ever present” (Mearsheimer 2018, 270, n. 52). Whether it is recognized as a state officially or symbolically by other states also matters little. Basically, a state is a group of people on a piece of land with a sovereign ruler above them.

The strongest states are the great powers of the system. They muster “sufficient military assets to put up a serious fight in an all-out conventional war against the most powerful state in the world” and, for post-World War II great powers, possess “a nuclear deterrent that can survive a nuclear strike against it” (Mearsheimer 2014, 5). Consequently, great powers are states that appear in the historical record as possessing enough military capabilities to hold their own against other great powers. States important in terms of economy or culture but unable to generate large-scale military power are excluded. The number of great powers informs the polarity of the system, which can be unipolar (one great power), bipolar (two), tripolar (three), or multipolar (four or more).

Potential Hegemon

This study distinguishes between systems that have a potential hegemon and those which do not. Systems having a potential hegemon are indicated as ‘unbalanced’ throughout the dataset. This allows determining whether the distribution of power among the poles is relatively even or not. Indeed, IR scholars often attribute great explanatory power to the balance of power among the great powers (Mansfield 1993). A potential hegemon is a great power “powerful enough to plausibly establish hegemonic control [and] that is the most powerful within a region by a considerable margin” (Colby 2021, 8). More specifically, it

must have excellent prospects of defeating each opponent alone, and good prospects of defeating some of them in tandem. The key relationship, however, is the power gap between the potential hegemon and the second most powerful state in the system: there must be a marked gap between them. To qualify as a potential hegemon, a state must have — by some reasonably large margin — the most formidable army as well as the most latent power among all the states located in its region. (Mearsheimer 2014, 45)

However, Mearsheimer’s (2014) usage of the concept is confusing because he sometimes applies it at the regional level and sometimes at the systemic level . For example, he describes the Cold War as a balanced bipolar system but considers the Soviet Union a potential hegemon in Europe and Asia. Elsewhere, he refers to the current system as unbalanced, with the United States as the strongest pole (Mearsheimer 2018, 265–6, n. 2), but sees China as a potential hegemon in Asia (Mearsheimer 2010; 2019b). Therefore, whether the main cause of war is system-wide imbalance or imbalance at the regional level remains unclear. To circumvent the problem, I consider that hegemony before 1860 is system-wide — because the self-contained systems of old have the scale of today’s regions — while I consider hegemony after 1860 as applying only at the regional level — because no state was ever strong enough to qualify as a potential or actual global hegemon.³

Methodology

The scope of the dataset enables testing the main international relations approaches within a least-likely design (see Levy 2008). A majority of the IR corpus is explicitly or implicitly based on modern Western experience. Most theories are thus likely to apply well to modern Western cases but poorly to ancient or non-Western cases. Hence, existing theories that can explain ancient or non-European cases well will be strongly reinforced. I do not attempt here to test any specific set of hypotheses. Instead, this article has an inductive flow: I first present the data and draw implications at the end of the piece.

Whenever possible, I used relevant data collected by international relations scholars, such as Hui (2005), Kaufman, Little, and Wohlforth (2007), Levy (1983), and Mearsheimer (2014). Otherwise, I referred to secondary historical sources. Great powers often form, rise, or decline over decades; thus, giving precise entry and exit years may seem arbitrary, but the long time span of the dataset mitigates this problem. In the last part of this article, I contrast the different power distributions with the frequency and duration of central wars. After linking these results with IR theories, it turns out that only classical and offensive realism appear relevant.

Mesoamerican System (1428–1518)

In 1428, the Aztec Empire formed by the confederation of Tenochtitlan, Texcoco, and Tlacopan, and the destruction of the Tepanec Empire.⁴ The other great powers were Chalco, Cholula, Coixtlahuaca, Huexotzinco, Michoacán, Metztitlán, and Tlaxcala. It was a balanced multipolar system. The Aztecs conquered Coixtlahuaca in 1458. After a long war, Chalco fell too in 1465. These victories and other conquests of wealthy territories toward the south throughout the 1450s and 1460s allowed the Aztec Empire to develop as a potential hegemon by the time Axayacatl took power in 1469.

Soon after accessing the Aztec throne, Moctezuma II unleashed a central war in 1503 by attacking Huexotzinco. Cholula and Tlaxcala soon joined in against the Aztecs. The Huexotzincans switched sides for a few years before allying with Tlaxcala again. However, Tlaxcala ended up annexing Huexotzinco in 1518. When Hernán Cortés and his contingent landed in Central America in 1519, the Tlaxcalans were quick to befriend the Spaniards, and the Spanish-Tlaxcalan alliance utterly destroyed the Aztec Empire in 1521. However, by this time, the Mesoamerican system had become a part of the Euro-Mediterranean international system (Carrasco 2012; Gruzinski 1992, chaps. 2–3; Hicks 1979; Isaac 1983a; 1983b; Jones 2007).

Hawaiian System (1738–1818)

A long period of all-out war came to an end with the Treaty of Naonealaa in 1738. Hawaii conquered east Maui in 1759; this and its superior resources made it a potential hegemon. A first central war started when Hawaii attacked Maui in 1778; after repulsing this offensive, Maui attacked Oahu in 1779. Benefitting from Hawaiian inner conflicts, Maui started another major conflict by attacking Hawaii in 1782 and conquered Oahu in 1783. With the annexation of Oahu and the reconquest of east Maui by the Mauians and Hawaiian domestic turmoil, Hawaii was no longer a potential hegemon, and the system turned to tripolarity.

In 1790, the Hawaiian leader Kamehameha attacked Maui. Kauai came to Maui’s help. This central war ended in 1791. As Kamehameha consolidated his power, grew his military, and decisively weakened Maui, it is safe to consider Hawaii a potential hegemon after 1791. Kauai attacked Maui in 1794 and was defeated, but Hawaii soon invaded Maui territory too and definitively destroyed Maui during the Battle of Nu uanuin in 1795. The system then turned to unipolarity, as Kauai could not compete with Hawaii’s enlarged resources. The end date of the Hawaiian international system can be placed in 1819, as Hawaii became more and more intertwined with the great powers of the soon-to-be global system (D’Arcy 2003; 2018, chaps. 2, 4–6, annex 1; Haas 1970, 105–6).

East Asian System (636 B.C.–1859 A.D.)

By 636 B.C., East Asia had two great powers: Chu and Jin.⁵ These two states had relatively equal capabilities. They faced each other during five central wars: the War of Chengpu (634–632), the War of Bi (608–597), the War of Zheng (585–581), the War of Yanling (575–571), all started by Chu, and the War of Zhanban (558–557) started by Jin. Qi, Qin, and Wu were ascending powers; the Xiangxu Peace Summit of 546 can be used to mark the transition from bipolarity to multipolarity.

Yue became a great power after military successes against Wu in 482. Yue destroyed Wu in 473. Jin collapsed to inner conflicts in 453. Wei became a great power in 406 after conquering Zhongshan. Han and Zhao became great powers too in 404, after a war with Qi. In 391, the Han-Wei-Zhao alliance attacked Chu and Qin, beginning a central war. Another central war started in 354 with a Zhao attack on Wei; Qi and Wei joined Zhao while Han supported Wei. The war ended in 352.

Qi seriously weakened Wei in 341; with its larger population and its wealth, Qi became at this occasion a potential hegemon. Chu destroyed Yue in 333. In 318, Wei led a coalition of Han and Zhao to attack Qin, but Qin won in 317. Another war was started by Qin, allied with Han and Wei, against Chu and Qi between 312 and 311. Chu and Qin invaded Han together in 303. Han was helped by Qi and Wei, which won in 302. Qi attacked Qin in 298 along with Han and Wei and won in 296. Qin annihilated Wei and Han as great powers in 293.

Qi lost its polar status in 284 in a central war during which it was invaded by a coalition of Yan and Zhao led by Qin. Yan’s military successes turned it into a great power. Benefiting from this collapse of Qi power, decades of extensive reforms, and territorial growth, Qin appeared as the new potential hegemon of the time. Zhao’s army was utterly crushed at the Battle of Changping in 260. Thus, after 260, no state had the military capabilities to endanger Qin: the system was now unipolar (Hsu 1999; Hui 2005, chap. 2, appendix II; Lewis 1999; Motin 2020, 106; Yuan 2018, 218–22).

The Han’s authority had totally collapsed by 196 A.D. when the emperor came under the protection of Cao Cao. This ended the unipolar period and ushered a multipolar era. The great powers were Cao Cao, Shu, Wu, Yi, and Yuan. In 207, Cao Cao conquered Yuan and became the dominant power of northern China; it was thus a potential hegemon. In 208, Cao Cao invaded Shu, but Shu and Wu won at the decisive Battle of Red Cliffs; this central war ended with Cao Cao’s defeat in 209 (Eikenberry 1994; Major and Cook 2017, chap. 12; Twitchett and Fairbank 1986).

In 214, Shu conquered Yi; the system was now an unbalanced tripolar one, with Cao Cao still being a potential hegemon. Shu assaulted Cao Cao in 219. Wu used the distraction to attack Shu. Yet, Wu shifted sides, and Wei (formerly Cao Cao) declared war on it. This major war dragged on up to 234. Wu attacked Wei in 253, soon followed by Shu. This ended with a draw in 258. In 263, Wei invaded Shu. Wu failed to help Shu. When Shu surrendered in 264, Wei was left utterly dominant, thus marking a transition from tripolarity to unipolarity (Motin 2022b).

Civil war in Jin China collapsed central authority by 316, and unbalanced multipolarity replaced unipolarity; Jin was still far stronger than any other of the great powers, which were Cheng Han, Dai, Former Liang, and Former Zhao. Later Zhao emerged in 319 and extinguished Former Zhao in 329. Former Yan became a great power in 337. Jin destroyed Cheng Han in 347. Former Qin (Zhao) conquered Yan in 370 and Dai and Liang in 376; the system thus became a balanced bipolar one. Qin invaded Jin in 378 but was utterly defeated in 383. Its power subsequently crumbled, and the system came back to unbalanced multipolarity.

In addition to Jin and a weakened Former Qin, the new great powers were Later Qin and Later Yan. In 391, Goguryeo became a great power after territorial conquests and internal reforms. Later Qin destroyed Former Qin in 394 and became known as Western Qin. Xia appeared in 407, Northern Wei in 409, and Northern Liang in 420. N. Wei attacked Xia in 426, starting a major war. N. Liang attacked Western Qin soon after. Xia allied with N. Liang and N. Wei with Qin. Meanwhile, Song (formerly Jin) assaulted Wei. Xia destroyed Western Qin just before being itself destroyed by Wei, which also repulsed Song’s army in 431.

N. Wei and Goguryeo conquered N. Yan in 436. N. Wei finally conquered N. Liang in 439. With northern China reunified under Wei and Song resurgent in the south, the system was now a balanced bipolar one, as both states were evenly matched. The first central war was a Song invasion of the North in 450, which ended in defeat two years later. Song was on the decline, and some wealthy provinces made secession to join Wei. After 469, the system thus became unbalanced. The next major war started in 495 with a Wei attack and stopped in 498. Facing a series of inner challenges, Northern Wei collapsed in 534 (Corradini 2006; Gernet 1982, chap. 9; Graff 2002, chap. 2–3, 6; Lee 1984, chap. 3; Lewis 2009, chap 3; Motin 2020, 111).

The new tripolarity was formed of Eastern Wei, Liang, and Western Wei. At first, Eastern Wei had a large military lead, making it a potential hegemon. The first major war broke out in 547 when Liang and then W. Wei entered E. Wei territory. Liang collapsed in 555 and was replaced by Chen, which managed to hold against Northern Qi (formerly E. Wei) and W. Wei offensives; the war ended in 556. Yet, due to Northern Zhou’s (formerly W. Wei) territorial conquests, internal reforms, and N. Qi’s military decline, the power gap between N. Qi and N. Zhou was almost closed by 560. The next central war began when N. Zhou invaded N. Qi in 576. It managed to conquer N. Qi and push back a Chen attack in 577. From then on, with its accumulated resources, N. Zhou became the unipole of the system (Motin 2022b). However, this unipole disintegrated in 907.

The new multipolar order had Balhae, Chu, Jin, Later Liang, Shu, Southern Han, Wu, and Wuyue as poles. Liao became one too in 916. In 923, Jin conquered Later Liang and became Later Tang, starting a series of weak regimes in South China. South China conquered Former Shu in 925. Liao decisively beat Balhae in 926. Shu came back to life in 934, and Goryeo unified the Korean Peninsula in 936. Southern Tang (Wu) annexed Chu in 951. By 960, when the Song dynasty took power in South China, the difference of power between Liao and Song on one side and the smaller states on the other made clear that the system had become bipolar (Clark 2009; Gernet 1982, chaps. 14, 16; Hung 2014; Lau and Huang 2009; Lee 1984, chap. 5; Smith 2009; Standen 2009).

The first two central wars of this bipolarity were Song attacks on Liao in 979 and 986. In 1004, Liao attempted to terminate Song but was defeated the next year. Peace lasted more than a century but ended when the Jin replaced the Song in 1125. They immediately attacked Song, and this war lasted up to 1142. The next major conflict was also a Jin attack on Song in 1161, which ended in 1165. The final war was a Song invasion in 1206, which was repulsed in 1208 (Motin 2020, 111–2; Wang 2013; Yuan 2018, 222–6).

At the same time, the Mongol Empire was enjoying a meteoric rise. It started its conquest of Jin in 1211, thus signaling the end of bipolarity and the coming of tripolarity. The invasion of Jin lasted until its destruction in 1234; the Mongols had the help of the Songs, who were eager to conquer Jin too. From then on, with their superior army and the resources of their vast empire, the Mongols were the potential hegemon in a bipolar system. They attacked Song a first time from 1235 to 1242. The second major offensive unfolded from 1258 to 1259. The third in 1268 was the last; the Song surrendered in 1276, leaving a unipolar system behind (Craughwell 2010, chaps. 7–8, 15–6; Davis 2009a; 2009b).

When Japan unified in 1590, it suddenly found itself with a large and competent army. The system thus turned to unbalanced bipolarity, as China was still far stronger than Japan. Indeed, although Japanese troops were superior in quality, China had larger numbers, superior artillery, and was wealthier and more populous. Japan invaded Korea in 1592 and hoped to take down China. However, this enterprise failed in 1598, and Japan’s power subsequently collapsed in 1600 due to inner strife (Hawley 2005; Turnbull 2008). The system thus turned back to unipolarity.

During the nineteenth century, European great powers became more and more present in Asia. By the end of the Second Opium War (1860), it was clear that the international systems of Asia and Europe were irremediably connected. In consequence, 1860 is used to mark the end of the East Asian system and the beginning of the global system (Gernet 1982, 574–80; Motin 2022a, 138; Suzuki 2019, 1101).

Near East System (744–539 B.C.)

The period from 744 to 710 B.C. was an unbalanced multipolar system with Assyria as a potential hegemon.⁶ At the beginning of the period, the great powers were Arpad, Assyria, Babylonia, Carchemish, Damascus, Egypt, Elam, Hamath, Samaria-Israel, and Urartu. Tiglath-pileser III ascended to power in 744 and reformed Assyria extensively. Assyria accordingly became a powerhouse ready for expansion. After a first war, it annexed Arpad in 740. After another war, Assyria conquered Damascus and part of Samaria-Israel in 732. Babylonia was also seized the same year. However, benefitting from Assyrian dynastic instability, Arpad, Babylonia, Damascus, and Samaria-Israel came back to life in 721.

The central war that decided the system’s fate raged from 721 to 713. Assyria assaulted a coalition led by Babylonia and Elam, which managed to hold their ground. In the West, Assyria defeated an anti-Assyrian coalition of Arpad, Damascus, Egypt, Hamath, and Samaria-Israel. Arpad, Damascus, Hamath, and Samaria-Israel all became part of Assyria. Carchemish was destroyed in 717. War against Urartu and its allies raged until its defeat in 713, when it was reduced to a secondary power.

The destruction of Babylon in 710 marks the switch to unipolarity because Assyria became too strong to be balanced. But Assyria began to decline in the second part of the seventh century, and unipolarity ended with the independence of Babylonia in 626. The other powers were Egypt, Lydia, the Medes, and Urartu. Babylonian independence led to a central war lasting from 626 to 605. Babylonian and Median powers rose extremely fast. Egypt allied with Assyria against Babylonia and the Medes. Assyria began to lose ground and was finally destroyed in 610. Urartu was neutralized by the Babylonians close after. In 605, they beat the Egyptians out of the Levant, ending this major war.

After Cyrus the Great took power in 550, the system collapsed in a matter of years as Persia expanded exponentially. The two decades that followed were thus unbalanced. Lydia was conquered in 547, and unbalanced tripolarity existed for a few years. The fall of Babylon in 539 marks the extension of the system toward the East Mediterranean region (Boardman et al. 1991; Kaufman and Wohlforth 2007, 23–40; McEvedy 1967, 46–50; Van De Mieroop 2016, 265–300; Wise 1981, 12–34).

Eastern Mediterranean System (538–202 B.C.)

Persia was the first power of this unbalanced multipolarity, followed by Athens, Corinth, Egypt, and Sparta. The most remarkable event of the late sixth century was the Persian annexation of Egypt in 525. After the Ionian Greeks revolted in 499 and gained support from the European Greeks, King Darius vowed revenge. A central war started when the Persians launched an all-out attack in 480, nearly dominating Greece. Nevertheless, the Greek victories at Salamis and Plataea in 479 ended Persian ambitions (Boardman et al. 1988, parts 1–2; Van De Mieroop 2016, 308–13). This date marks the end of Persia’s unbalanced power since Athens became strong enough to take on the Persian Empire (Kroenig 2020, chap. 4).

Balanced multipolarity was the norm from 478 on. Persia was still a great power as well as Athens, Sparta, and Corinth. Athens, emboldened by victory in 479 and its growing power, confronted Persia in Asia Minor, Egypt, and Cyprus. Sparta grew wary of Athens’s rising star and started with Corinth a central war in 431, the Archidamus War, which ended with an Athenian victory in 421. A second major conflict, the Decelean War, started with a Spartan attack in 414. This time Sparta and its allies Corinth and Persia won in 404, and Sparta became the dominant power in Greece (Lewis et al. 1992, chaps. 2–6, 9–11; Motin 2020, 106–8).

The weakening of Athens after its final defeat in 404 left enough space for the rise of Thebes. Moreover, Egypt recovered independence from Persia that same year and was able to fight Persia off until 343. The Corinthian War saw Sparta defending its newly acquired dominant position in Greece against a coalition of Athens, Corinth, Thebes, and Persia from 395 until 387. This central war ended in a relative Spartan victory. Meanwhile, Persia was going through domestic turmoil. While Greek cities were exhausted by constant warfare and Persia weakened, in 359, Philip II became king of Macedonia and engaged in self-reinforcement and expansion: this was the start of an unbalanced multipolarity.

A final central war unfolded from 340 to 330. As Philip wanted to unite Greece behind him for preparing his war against Persia, he found himself at war with Athens in 340, soon joined by Thebes and Corinth. The Greek city-states surrendered in 337. Philip declared war on Persia and began to prepare for the invasion of Asia. When his son Alexander took power, he had to face a revolt of Athens and Thebes. Thebes was annihilated in 335, and Athens bandwagoned with Alexander. Persia ended up totally defeated in 330. The death of Alexander in 323 led to the division of his empire. Thus, the system can be considered a Macedonian unipolarity from 329 to 322 (Lewis et al. 1994, chaps. 2–4, 6–8, 14–7; McEvedy 1967, 52–8).

At the beginning of the new multipolar period, there were five powers: Anatolia, Egypt, Macedon, the Seleucid Empire, and Thrace. Alarmed by the conquests of Anatolia, the other great powers asked that it relinquish its last acquisitions. It refused and passed on the offensive. The Third War of the Diadochi, the Babylonian War, and the Fourth War of the Diadochi form a single central war (314–301). Antigonus of Anatolia was defeated and killed in 301. This multipolar period ended in 281 with the Battle of Corupedium and the destruction of Thrace (McEvedy 1967, 60–3; Walbank et al. 1984, chaps. 2–7).

What followed was a balanced tripolar period populated by Macedon, Egypt, and the Seleucid Empire. The First Syrian War started with the Seleucids attacking the Ptolemies (274–271). The Seleucids attacked again in 260, this time in company of the Macedonians; this Second Syrian War ended in 253. The Third Syrian War (246–241) started with a Ptolemaic attack on the Seleucids and ended with a clear victory for Egypt. Finally, the Fourth Syrian War (219–217) saw the Seleucid Empire invading Egyptian territory. The Seleucids met with defeat there too (Eckstein 2006, chap. 4; Hölbl 2001, 40–3; Kohn 2007, 154, 547; Thorne 2012, 118–24; Walbank et al. 1984, chaps. 11–2). Starting from 207, Egypt fell into growing turmoil. The Macedonians and the Seleucids ganged up on Egypt to eliminate it. The Egyptians, out of desperation, called the Romans to the rescue. This entry of Rome into the Eastern Mediterranean marked the end of that system and the emergence of a larger Euro-Mediterranean one in 202 (Eckstein 2006, 104–14).

Western Mediterranean System (272–202 B.C.)

The Western Mediterranean system of the second century B.C. was a balanced bipolar system centered on Carthage and Rome. After establishing its domination over Italy, Rome started to contest Carthaginian predominance. A first central war, the First Punic War, occurred from 264 to 241 at the initiative of Rome. In the aftermath, relations remained tense. Finally, in 219, Carthage decided to provoke war. After a long conflict — the Second Punic War — it met with defeat in 202. The end of this bipolar system, simultaneously with the collapse of Egypt, marked the emergence of a larger Mediterranean system (Copeland 1996, 64–7; Motin 2020, 108–9; Scullard 1989).

Euro-Mediterranean and Global Systems (201 B.C.–2016 A.D.)

This system was an unbalanced multipolar one with Rome, benefitting from superior administrative and military capabilities, as a potential hegemon. The other great powers at the beginning of the period were Egypt, Macedon, and the Seleucid Empire.

The Fifth Syrian War started in 201 when Macedon and the Seleucids invaded Egypt, and the Romans decided to intervene on Egypt’s behalf and against Macedon; it ended in 195 with a Roman victory against Macedon and a draw between the Seleucids and the Egyptians. But Rome and the Seleucid Empire were on a collision course, and the Seleucids invaded Greece in 192; Rome declared war and allied with Macedon. The Roman army pushed back the Seleucids, crossed into Asia, and brought defeat to the Seleucid Empire in 189.

The gains made during the Seleucid War elevated Pergamon as a great power in 188. Macedon was displeased by the growing Roman clout and started to rebuild its influence. After another losing war started in 171 by the Romans backed by Pergamon, it was disbanded in 168 in four independent republics by Rome. Egypt took the occasion to attack the Seleucids but lost; it was saved only by a Roman intervention (Derow 1989; Eckstein 2006, chap. 5; Errington 1989; Kroenig 2020, chap. 5).

By taking Mesopotamia from the Seleucids in 141, Parthia became a full-fledged great power while the Seleucid Empire was declining at a fast pace. Pergamon exited the list of great powers in 133 after it was bequeathed to Rome by its dead king (Habicht 1989; Overtoom 2019, 131–9). Pontus can be considered a great power in 113 when Mithridates VI assumed power. Similarly, Armenia became a great power under Tigranes II (95). Noticing the overstretching of the Roman army, Pontus took his chance and started the First Mithridatic War in 89. Parthia used the diversion to invade the Seleucid Empire. Sensing weakness, Armenia attacked the Parthians. Despite Pontus’s impressive initial successes against the Romans, the war ended in 85 by a draw. The Seleucid Empire disappeared from the map in 83 after being conquered by Armenia.

Pontus tried to roll back Roman expansion again in 74 with the help of Armenia. Parthia lent a hand to the Romans against Armenia. After a lengthy war, both Armenia in 66 and Pontus in 63 B.C. ended utterly defeated and exited the ranks of the great powers. As a result, Asia Minor and the Near East were annexed by Rome. By this time, Egypt was too weak to compete with Rome and was a Roman protectorate in all but name. The only great power left to challenge Rome was Parthia (Hind 1992; Hölbl 2001, 223–5; Mayor 2014; McGing 2009; Olbrycht 2009; Sherwin-White 1977, 72–5; 1992).

This new bipolar system was unbalanced, as Rome was largely superior to Parthia in almost all metrics. There were overall 14 central wars between Rome on one side and the Parthian Empire and its successor, the Sasanian Empire, on the other. Six of these wars were started by the Romans and eight by the Parthians/Sasanians. Centuries of war exhausted the two empires. The arrival of new actors and the division of the Roman Empire into two parts in 395 A.D. marked the end of this bipolarity and the start of balanced multipolarity (Motin 2020, 109–10).

This balanced multipolar system counted four great powers: Byzantium (the Eastern Roman Empire), the Hunnic Empire, Persia, and the Western Roman Empire. The Visigothic Kingdom can be considered a great power from 418. The Vandals completed their conquest of North Africa when they took Carthage in 439, becoming a great power. After the death of Attila in 453, the Hunnic Empire crumbled. Francia became a great power in 486. The fall of the Kingdom of Italy in 493 marked the end of the Western Roman Empire and the elevation of the Ostrogothic Kingdom as a great power.

Reinvigorated by Emperor Justinian (527), Byzantium became a potential hegemon. The Byzantines conquered the Visigoths in 534. In 535, the Byzantines invaded Italy and fought a protracted war against the Ostrogoths, thus starting a central war. The Franks soon entered the fray and attacked the Byzantines, while the Ostrogoths pushed the Persians to attack Byzantium. The Byzantines also invaded the Visigoths and conquered southeastern Spain. The Ostrogothic Kingdom was defeated in 553, and the war ended in 554 with a Roman victory.

The Avar Empire emerged as a great power in 567. The Byzantine military was stretched thin as a plague outbreak wreaked havoc throughout the empire. In 568, Lombardy became a great power and invaded Italy, thus putting an end to the Byzantine Empire’s standing as a potential hegemon. After a decisive defeat in front of Constantinople in 626, the Avars exited the ranks of the great powers (Cameron, Ward-Perkins, and Whitby 2000; Collins 1991, 75–126; Fouracre 2005; Heather 1995; Pohl 2018, chaps. 3, 7).

The Arab Empire became a full-fledged great power after it invaded Syria in 636. It destroyed Persia in 651 and can be considered a potential hegemon starting from 692, with the end of the Umayyad civil war. A central war started in 716 when the Arabs invaded Byzantium and besieged Constantinople. They also vanquished the Visigoths in Spain in 721 and then attacked southern Francia. Decisive Byzantine and Frankish victories and a major Berber revolt in North Africa led to a decisive Arab defeat in 740. The Umayyad dynasty subsequently collapsed due to a civil war in 750, ending its position as a potential hegemon. It was replaced by the Abbasid dynasty. The Emirate of Córdoba became a rump state of the Umayyads in Spain (Collins 1991, 135–43, chap. 10; Hoyland 2015).

Charlemagne destroyed Lombardy in 774. Bulgaria emerged as a great power in 805 after extensive conquests against the Avars. The Treaty of Verdun in 843 divided the empire of Charlemagne and gave birth to a new great power, Germany (or East Francia). The Rus also appeared as a great power in 882. The Fatimid Caliphate became a great power in 969 after conquering Egypt. A string of defeats against the Fatimids and the Byzantines ended the decaying Abbasid Caliphate’s standing as a great power that same year. Córdoba started to crumble due to a major civil war in 1010. The Byzantines annihilated Bulgaria in 1018 (Collins 1991, chaps. 15–9; Jordan 2001, chap. 5; McKitterick 1995; Reuter 1999).

The death of Yaroslav the Wise in 1054 led to the collapse of the Rus into numerous small states. England can be considered a great power after the 1066 Norman conquest. The Seljuk Empire became a great power after the 1071 Battle of Manzikert against the Byzantines. Morocco appeared as a great power after it started its conquest of Spain in 1086. The Seljuk Empire internally collapsed after the death of its ruler Sanjar in 1157.

Saladin took over the Fatimid Caliphate and invaded Jerusalem, prompting in 1189 the Third Crusade led by England, France, and Germany. Byzantium at first fought the Crusaders before letting them pass through its territory. It ended in 1192 with the victory of the crusading states. The Fourth Crusade ended Byzantium’s great power stature by rampaging Constantinople in 1204.

The Mongol Empire was a potential hegemon from 1240, when it conquered Russia, up to 1260, when it plunged into civil war. The Golden Horde in Eastern Europe and the Ilkhanate in the Middle East succeeded it. The death of Emperor Frederick II in 1250 led to the political fragmentation of the Holy Roman Empire — Germany — which thereafter cannot be counted as a great power. Morocco disappeared in 1269 after its conquest by a Berber dynasty (Abulafia 1999; Craughwell 2010, chaps. 12–5; Fromherz 2010, 1–17; Luscombe and Riley-Smith 2004; Peacock 2015; Riley-Smith 2004).

The Ilkhanate disintegrated in 1335. The Ottoman Empire can be considered a great power after the 1389 Battle of Kosovo. The Timurid Empire was a great power by 1394, as it dominated the Middle East. The Golden Horde was severely weakened when Timur ravaged it in 1395; however, the death of Timur in 1405 led to the collapse of his empire. Austria became a great power after acquiring Burgundy in 1477. Spain emerged as a major force in Europe with the union of Aragon and Castile in 1479 (Allmand 1998; Jones 2000; Wing 2007).

The Mamluks (Egypt) disappeared in 1517 after being conquered by the Ottomans (Goldschmidt and Davidson 2010, chap. 9). In 1519, by marriage, Austria and Spain united into a single state. France started a major war against Austria-Spain in 1521. England joined on the Spanish side. The war ended in 1526 after a decisive French defeat at Pavia. The French allied with the Ottomans and started a new central war against Austria-Spain in 1542. Once again, the English intervened on Spain’s behalf. This war ended in 1546. In 1551, France attacked the Austro-Spaniards one last time again with Ottoman support. Austria and Spain divided after the death of Charles V in 1556. The English joined once more against France; the war ended in 1559. The Netherlands achieved independence and became a great power in 1579.

France plunged into trouble during the 1560s, and the Ottoman Empire took the road of decline during the 1570s: when Spain annexed Portugal in 1580, it became a potential hegemon. A central war started in 1585 when Spain tried to re-impose its control over the Netherlands, which the English supported. Spain also rampaged throughout France, but it was finally defeated in 1609.

Sweden emerged as a great power after a decisive victory against Russia in 1617. Another central war, the Thirty Years’ War, started in 1618 when Ferdinand II tried to impose Catholicism throughout the Holy Roman Empire. It soon drew in Spain while the Netherlands, England, Sweden, and France hoped to tear down Spanish and Austrian power. The conclusion of the war in 1648 marked the end of Spain’s bid for hegemony (Beller 1970; Elliott 2002, chaps. 7–9; Lee 1984, chap. 9; Levy 1983, chap. 2; 1985; Motin 2020, 112–4; Trevor-Roper 1970).

Spanish power collapsed while French power was growing. When Louis XIV ascended to the throne in 1661, he found a France that was a potential hegemon. The first of his three central wars started when he invaded the Netherlands in 1672. Austria, England, and Spain eventually supported the Netherlands, while Sweden helped France to prevail in 1678. The War of the League of Augsburg started in 1688 with a French attack toward Germany; Austria, England, Netherlands, Spain, and Sweden were defeated in 1697. The Treaty of Karlowitz in 1699 put an end to the Ottoman Empire as a great power. The War of the Spanish Succession began in 1701 when Austria contested French domination over Spain. Vienna was soon supported by the English and the Dutch. The war ended in 1713 by a draw. The Netherlands exited the ranks of the great powers on the same occasion, while France was not a potential hegemon anymore (Levy 1983, chap. 2; 1985; Motin 2021).

Sweden was decisively defeated by Russia in 1721. By the same token, Russia became a great power. The following central war (War of Jenkins’s Ear/Austrian Succession) started in 1739 when Britain declared war on Spain due to colonial competition. Prussia became a great power after it attacked Austria and occupied Silesia in 1740. France and Spain supported Prussia while Russia joined Austria. This major war ended in 1748. The Seven Years’ War came soon after when Britain attacked French colonies in North America in 1755. Prussia allied with Britain, while Austria, Russia, and Spain allied with France. The British and Prussians eventually won in 1763 (Browning 1993; Levy 1983, chap. 2; 1985; McKay 1983, chap. 6; Nester 2000).

France became a potential hegemon in 1793 due to the quick growth of its military power. The French Revolutionary and Napoleonic Wars started in 1793 when Paris declared war on the coalition of European powers. Spain was conquered by Napoleon in 1808. War engulfed all the great powers and lasted until Napoleon’s final defeat in 1815. France was not a potential hegemon anymore after that.

Italy became a power after it unified in 1861. Japan defeated China in 1895 and rose to great power standing. The United States became a great power thanks to its victory against Spain in 1898. Germany was a potential hegemon in Europe in 1903, as it had accumulated military forces far stronger than its rivals. Austria started World War I in 1914. All the great powers participated in the war. Austria disintegrated due to the defeat of 1918, while Germany lost its potential hegemon status.

Nevertheless, the Germans made a comeback in 1939 by rebuilding their military and started World War II right after. Japan, too, became a potential hegemon in Asia in 1940. France was beaten by Germany that same year. Italy was vanquished by the Allies in 1943. Germany and Japan exited the ranks of the great powers in 1945 after losing World War II. Britain, too, exhausted, exited the ranks of the powers.

The system became an unbalanced bipolar one. Only the Soviet Union and the United States were left standing. The Soviet Union, with its massive army, was a potential hegemon in both Asia and Europe between 1946 and 1991. Yet, it disintegrated in 1991, leaving the United States as the sole great power up to 2016 (Levy 1983, chap. 2; 1985; Mearsheimer 2014, chaps 6–9; 2019a).

Balanced multipolar systems are the least prone to central wars. Unbalanced multipolarity and bipolarity are more violent, both close to a central war every 30 years. There is no major difference between balanced and unbalanced bipolarity. The most violent type of system is tripolarity, regardless of the distribution of power within it.⁷ Furthermore, unipolarity has been rare outside the East Asian system (Table 1).

The Western Middle Ages — traditionally defined as the 476–1492 period — is an outlier, with only three central wars. If periods with a potential hegemon are excluded, it saw only one central war for nearly nine centuries of data. But even without the medieval era, multipolarity still produced only one major war every 78 years, thus being the most peaceful type of structure.⁸

The proportion of peacetime to wartime tells a similar story (Table 2); tripolarity sees around 2.5 years of peace for one year of central war, and unbalanced multipolarity is close to three for one. Central wars occupy less time in unbalanced bipolar systems than in balanced bipolar ones. Periods of peace are far more prevalent in balanced multipolar systems.

The longest central wars occur under multipolarity. Tripolarity — balanced and unbalanced — and balanced bipolarity witness wars lasting on average around seven years. It seems that the more poles there are, the longer the wars. The shortest wars happen in unbalanced bipolar systems (Table 3). We can hypothesize that because one side is markedly weaker and with no possibility of finding allies, it is likely to be destroyed or give up more rapidly.

The dataset has 25 distinct cases of potential hegemons. Only seven of them succeeded in becoming an actual unipole. The results are thus nuanced; although balance of power as an outcome is not automatic, balances tend to survive more than two times out of three. The potential hegemon was among the initiators in around half of the central wars that occurred within unbalanced systems. Overall, concentration of power — horizontal and vertical — is prone to devastating wars (tripolarity, bipolarity, and unbalanced multipolarity). On the contrary, equal and wide dispersion of power like in the Middle Ages is more peaceful. Central wars are rarer where power is dispersed between numerous poles evenly matched.

Although the dataset does not describe overall violence but only central wars, these results have implications for the main ‘isms’ of international relations theory.⁹ The data do not support institutional, cultural, and discursive theories — mainly constructivism and liberalism. The distribution of power appears correlated to the occurrence of central war, regardless of the domestic or cultural conditions. The dataset does not support defensive realism either, for great power aggression to overturn the status quo is widespread and bipolarity, contrary to Waltzian expectations (Waltz 1979, chap. 8), is not a pacifying structure. However, it supports offensive and classical realism, which points out that power imbalances tend to produce major wars, that states try to reach hegemony, and that aggression sometimes pays off (Table 4).

Therefore, these results also contradict those who believe that realism is an oversimplistic, ahistorical, and Eurocentric approach unable to account for non-modern cases (e.g., Buzan and Little 2009). One can only agree with Waltz (1979, 66) that the “texture of international politics remains highly constant, patterns recur and events repeat themselves endlessly.”

What about structural durability (Table 5)? Tripolar systems are the shortest lived, followed by unbalanced multipolarity. Bipolar and balanced multipolar systems are relatively stable; because there is no obvious pattern here, this requires further research. It at least confirms Waltz’s (1979, 170–83) expectation that bipolar systems could last. The most stable structure is unipolarity. This gives ammunition to those who argue that unipolarity is durable (Hansen 2011, chaps. 2–3; Wang 2020; Wohlforth 1999).

All in all, this evidence is more crippling for some IR schools than for others. First, it would be unfair to blame liberalism and constructivism for faulty predictions because they actually do not predict anything. They are more collections of assumptions and mid-range theories than testable grand theories. Second, my data only accounts for the distribution of power among independent states. To test in a more deliberate way liberalism, constructivism, and other approaches, one would need to collect data about their preferred causal variables (domestic regimes, trade, interest groups, cultural settings, etc.). The evidence here provides more of a challenge than an outright rebuttal.

However, it is far more damning for defensive realism. This work took at face value scholars who distinguish between international politics and foreign policy and affirm that neorealism is not falsified by observing the actions of particular states (Waltz 1996). Waltz recommended testing neorealism only in terms of international outcomes over the long run and not by looking at what states do on a day-to-day basis. Accordingly, I ignored the behaviors of the states and solely accounted for outcomes. Taken in its own terms — over the long run, at the systemic level — defensive realism has little to say about the workings of the world.

This study’s main limitation is to focus only on central wars. Further research beyond the outbreak of central wars and exploring more limited wars between great powers or even including wars between minor powers would prove invaluable. Engaging more intensively with world history is indeed one of the most fruitful ways to develop and test international relations theories.