Yet they themselves were not easily conquered: due to the lack of preexisting state structures, individual polities often had to be subdued one by one and at increasing remove from naval supply routes, and provincial administration needed to be established from scratch. This helps account for the long duration of Roman campaigning in North Italy and especially in the Iberian peninsula in the second century BCE.26
Later, overwhelming concentrations of force produced faster results, when Caesar took control of Gaul in less than a decade and Augustus’s forces secured the entire Danube basin. Even so, subsequent Roman setbacks in Germany (9–16 CE) highlighted the limits of progress into ecologically marginal tribal areas.27
From a Roman perspective, much of Europe, exposed to unilateral imperial aggression and expansion, was a ready-made periphery—and one whose formidable challenges were almost perversely well suited to a military system that was not only capable but even desirous of sustained and not always profitable warfare over long periods of time. Rome’s advantage was thus just as deeply rooted in its oligarchic politics and cooperative institutions as in its mobilization capabilities.
CORE AND PERIPHERIES COMPARED
The regions into which Rome came to expand varied considerably with respect to political organization and economic development. Figure 3.4 maps some key differences in ideal-typical terms, following Michael Doyle’s taxonomy. Tribal peripheries lacked centralized state institutions and were characterized by low differentiation and high communal loyalty; patrimonial monarchies were large but lacking in political integration and the ability to mobilize their far-flung populations; fractionated republics were equipped with central state institutions but contained divided communities with factional loyalties.28
In this diverse environment, Rome enjoyed significant advantages relative to all its competitors. I focus on three key variables: intensity, scale, and integrity (table 3.1). Intensity is a measure of depth, expressed by the military participation rate. Scale measures breadth, in this case the size of the population to which a given level of mobilization intensity can be attributed. In the absence of major technological imbalances, military capacity can be defined as the product of intensity times scale. This capacity was critically mediated by integrity, in particular the degree of cohesiveness among the decision-making ruling class and more generally the stability of the polity.
FIGURE 3.4 Stylized typology of peripheries in the Roman-era Mediterranean.
Around 200 BCE, the Roman-Italian state and alliance system in peninsular Italy was the only entity anywhere in the world that scored high on all three metrics. Its military mobilization capacity was high and extended over several million people. Although territorial gains in Sicily and the Iberian peninsula reduced the average per capita military participation rate (hence the parenthetical “medium” ranking in table 3.1), conditions in the intensely militarized Italian core remained unaffected. Aside from some defections of allies during Hannibal’s invasion, integrity was generally high and oligarchic consensus was (still) solid. Moreover, Rome’s advantages in the other two categories were so large that it was able to score victories over external enemies even once its integrity was compromised later on (in the 80s and 40s BCE).29
By contrast, the continental tribal periphery was highly militarized but suffered from poor coordination: the latter was its key weakness. The Greek Syracusan state in Sicily depended on expensive mercenaries to raise its intensity to levels that were high relative to its midsize population but was constrained by demography and intermittent political volatility. In this case, population size was the decisive disadvantage. Mainland Greeks combined intermediate levels of intensity with intermediate scale: as a result, military manpower was insufficient to challenge Rome. Carthage, albeit endowed with a cohesive core and integrity, and roughly Rome’s equal in terms of total population number, did not maintain high mobilization levels for as large a share of the people under its control as Rome did. The Hellenistic kingdoms score high on scale but medium (Macedon and the Ptolemies) or even low (Seleucids) on intensity, and from high (Macedon and, at least for a while, the Ptolemies) to medium (Seleucids and later Ptolemies).30
None of these societies were capable of adapting rapidly to meet the Roman challenge: Rome’s rise to power in Italy in relative isolation from its future opponents had delayed the latter’s response, and, more importantly, deep-seated structural differences militated against a shift toward Roman-style mobilization on a large scale: the fractiousness of Greek polities, Carthage’s and the Macedonian conquest elites’ alienation from their imperial subjects, and tribal units’ inability to scale up stable cooperation.
On top of everything else, Rome’s consistently high scores were reinforced by geopolitical constraints on its competitors: Carthage, the runner-up in scores and not coincidentally Rome’s most tenacious adversary, operated for the most part on its own, geographically remote from potential allies in the eastern Mediterranean; and the Hellenistic kingdoms, trapped in long-standing mutual enmity, failed to balance against Roman pressure. Stronger ties between east and west or a more unified east—along the lines of the Ottoman empire much later—might well have made it more difficult for Rome to convert its structural advantages into successful expansion. Neither one of these constraints was particularly contingent, and no plausible conservative rewrite of history would produce a substantially different environment.31
ROME’S MEDITERRANEAN ADVANTAGE
Finally, one other factor merits attention. Rome attained naval security and then supremacy across the Mediterranean at a very early stage of its expansion beyond Italy. Rome’s naval defeat of Carthage in 241 BCE, which concluded their first round of warfare, left much of the Carthaginian empire intact but effectively ended its sea power. Thanks to the lack of other major seafaring societies west of the Aegean and Egypt, this made Rome the undisputed hegemon over more than half of the waters of the Mediterranean Sea. By the same year, the Ptolemies had achieved naval supremacy in most of the remainder of this sea basin, from the Aegean (up to Thrace and the Dardanelles) and throughout the Levant all the way to the Cyrenaica.32
Their capitals separated by well more than 2,000 kilometers of sea travel as well as by shared enemies or sources of concern, these two powers had been on friendly terms ever since Rome had first consolidated control over the Italian peninsula. In the third century BCE, the Ptolemaic empire maintained a large navy that included some of the largest warships built in all of antiquity. Its interference in Rome’s operations against Sicily and North Africa would not have been a trivial matter. As it was, cordial relations were maintained until the decline of Ptolemaic power in the second century BCE allowed Rome to extend its naval hegemony eastward simply by filling the vacuum this created while the second-tier navies of Macedon and the Seleucids were shunted aside without much difficulty.33
After 190 BCE, no Hellenistic state challenged Rome by sea. The true extent of its hegemonic status is reflected in the later spread of piracy: as the Roman state was able to neglect its naval capabilities because it had run out of state-level competitors, no other powers were able to step in to improve security. In the end, it was Rome that suppressed maritime raids by nonstate actors (or “pirates”) with overwhelming force.34
Precocious naval supremacy was a boon to Roman expansionism. It made it possible to deliver supplies in support of troops in distant theaters: historical accounts underline the crucial importance of maritime logistics in the wars against Carthage and the Seleucids. In a more abstract manner, a highly schematic model of state formation suggests that secure coastal borders lowered the cost of expansion. According to a crude computer simulation that divides Europe into uniformly sized territorial cells that engage randomly in conflict and vary only in terms of the cost of defending their borders—with coastal borders and major mountain ranges (Alps and Pyrenees) enjoying lower costs than land borders—coastal borders that are only moderately cheaper to defend than land borders yield outcome
s resembling the modern European state system. Conversely, substantially lower coastal defense costs favor larger-scale state formation.35
In this model, the lowering of coastal defense costs for only a single region—the Italian peninsula—in order to approximate Roman naval hegemony confers a considerable advantage. In this scenario, Italy, shielded by the Alps and long coastlines, tends to expand farther afield even if there is no variation at all in natural endowments, carrying capacity, and institutions. Several details merit particular attention. (1) Italy expands into Europe only if its coastal defense costs are massively lowered to less than one-third of those of other regions; otherwise there is no tangible effect at all. (2) Even under this specification, its coastal defense advantage works out reliably only if Italian expansion takes off early in the simulation, before less privileged rivals beyond the Alps have a chance to succeed. (3) Even in the most successful runs of the simulation does Rome fail to take over the Iberian peninsula.
All of this can readily be reconciled with actual historical developments. After 241 BCE, thanks to its naval hegemonic status, Rome’s coastal defense costs were vastly lower than others’; Italy was a pioneer in ancient European state formation; and unlike in the model, which recognizes only terrestrial connections, maritime connections helped it operate in the Iberian peninsula early on. The model requires a combination of two factors—very low naval defense costs and a head start (especially with respect to the western periphery of Europe)—to precipitate Italian (i.e., Roman) expansion across large parts of Europe. Those conditions precisely match the ones encountered by Rome from the mid-third century BCE onward.36
Naval supremacy was the first step in establishing an empire that was centered on the Mediterranean basin. A different and less crude model that captures historical conditions in greater detail illustrates that Rome reaped significant benefits from its undisputed command of this maritime inner line. In the absence of hostile interventions, the Mediterranean Sea functioned as a core of connectivity that facilitated the movement of people, goods, and information at very low cost and risk.
A geospatial network model of the Roman world (“Orbis”) that I developed together with Elijah Meeks visualizes the extent to which the coastal regions of the empire were much more interconnected—in terms of travel time and transportation cost—than their sprawling hinterland (figures 3.5 and 3.6): each unit of distance on the maps corresponds to a constant unit of cost.37
Not coincidentally, Roman expansion tracked this pattern quite closely: Romans first advanced into the most accessible regions (figure 3.7).38
More specifically, secure maritime connectivity afforded ready access to the capital-rich regions in the eastern Mediterranean that helped sustain costly expansion into non-Mediterranean Europe during the final stages of Roman conquest from the first century BCE to the second century CE.39
Roman mastery of the Mediterranean was unique: never again in history would one power exercise lasting control over its entire coastline, and its effective naval supremacy was not renewed until the days of Admiral Nelson, if not World War II. Moreover, the Roman dominions were unusual simply for being centered on the Mediterranean: among later sizable empires, only Habsburg Spain and the Ottomans shared this distinction, although on a smaller scale, especially the former. Neither one of them enjoyed anything like Roman hegemony.
FIGURE 3.5 Time cost of transfers from Rome (military; summer). Source: Scheidel 2014: 15, fig. 3.
FIGURE 3.6 Financial cost of transfers to Rome (cargo; summer). Source: Scheidel 2014: 22, fig. 8.
FIGURE 3.7 The Mediterranean core of the Roman empire. Source: Scheidel 2014: 23, fig. 10.
This is easy to explain. Even though a unified Mediterranean may have been a highly suitable core for an empire that already dominated it, later history documents the difficulties of reaching the requisite position of preeminence. This happened only once, at a time when lack of competition made it less challenging to establish hegemony over the less developed western half of the Mediterranean. Considering how much Rome struggled against just a single opponent during its first war with Carthage, a more crowded naval environment might well have curbed its naval expansion. Due to the subsequent spread of naval technology, this favorable situation was impossible to replicate later: no medieval or modern state could hope to attain hegemonic status by overcoming so few contenders. Competition had become too widespread.
Once they stepped beyond the Italian peninsula, Rome and its allies encountered unusually favorable conditions for military success and ongoing expansion. Closest to home, they faced an inner periphery of polities such as Syracuse, Carthage, and the Greek federations that could not match Roman mobilization intensity on a comparable scale. Farther away, kingdoms with extensive coordination capacity suffered from relatively low mobilization levels rooted in military technology, socioethnic stratification, and high costs. Conversely, small-scale stateless societies boasted high military participation rates but suffered from lack of coordination. During the formative stages of its overseas empire, Rome consistently outscored all its competitors on the critical variables of intensity, scale, and integrity. Weak naval development in the western Mediterranean laid the foundations for Rome’s precocious hegemony over the Mediterranean basin, allowing it to project power over great distances without having to worry about the security of its homeland. With the single exception of the initial resilience of the Carthaginian empire, it is hard to conceive of peripheries that would have been more vulnerable to Roman aggression.
CHAPTER 4
Counterfactuals
EMPIRE ABORTED?
I HAVE so far operated with the benefit of hindsight, defining core and periphery in keeping with actual historical outcomes. However, the world outside Italy was not merely a periphery-in-waiting ready to be conquered: it was also a potential source of threats to Roman state formation. Imperialism was not a one-way street. In studying what happened and trying to explain why it did, it is all too easy to err on the side of determinism—the notion that observed outcomes were the ones most likely to occur—or on the side of contingency—the notion that things might just as readily have turned out completely differently. Explicit consideration of counterfactual scenarios helps us chart a course between these extremes. The question is this: What were the odds that challenges from within or from outside could have derailed Roman expansion, preventing the creation of an empire that ruled 80 percent of Europe’s population for almost half a millennium?1
ITALY
Domestic developments that could have had this result were most likely to occur at very early stages of Roman state formation, which are extremely poorly documented. This makes it virtually impossible to judge the probability of alternative outcomes. Nevertheless, if early fifth-century-BCE Rome had for some reason not been the largest city-state in Latium, it would not have been able to assume a hegemonic leadership position. If social conflicts, dimly attested for the fifth and early fourth centuries BCE, had severely compromised the integrity of the Roman state, successful expansion might well have proved impossible. Given the quality of our evidence, there is simply no way of telling whether any of this might plausibly have occurred. However, for all we can tell, once we reach the phase of open-ended Roman expansion into the Italian peninsula from the second third of the fourth century BCE onward, truly crippling internal mishaps were much less likely to strike. Serious domestic conflict—discussed below—did not flare up until the early first century BCE, when much of the empire had already been put in place.2
A counterfactual perspective can be more fruitfully applied to external challenges and their potential impact. Given that Rome did not suffer more than brief setbacks once it had broken out of Latium, opposition from within Italy does not seem to have been a particularly strong candidate for changing the trajectory of Roman state formation. Unable to project power over longer distances, the Samnitic federation was not a credible alternative to Roman hegemony. And while
it may be true that had the Gauls who sacked Rome in the early fourth century BCE chosen to settle there, the Roman Republic would have ended there and then, such a decision would have been without parallel and is therefore not a plausible option either.
EASTERN CHALLENGES
In the most general terms, the most likely source of disruption lay to the east, in more developed parts of the Old World. Intervention or even absorption by a major power before Rome had fully extended its control over peninsular Italy would have posed the most serious obstacle to Roman imperialism. As I pointed out above, Italy was relatively remote from earlier state formation in the Near East. Even so, the risk of hostile contact gradually increased over time as the eastern political-military network expanded.
FIGURE 4.1 The Akkadian, Neo-Assyrian, and Achaemenid empires.
The odds of any such contact increased only very slowly. When the earliest empires arose in the Fertile Crescent, starting with Old Kingdom Egypt and Akkad in the third millennium BCE, there was no realistic way of their ever interfering in Italy. A more recent iteration, the Neo-Assyrian empire of the seventh century BCE, briefly reached into western Anatolia and Egypt but likewise never had a good chance of developing far enough to threaten Italy (figure 4.1).3
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