Book Summary: “The Arc of War” by Levy and Thompson


Title: Arc of War: Origins, Escalation and Transformation
Author: Jack S. Levy and William R. Thompson
Scope: 4 stars
Readability: 3 stars
My personal rating: 4.5 stars
See more on my book rating system.

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Topic of Book

The authors seek to explain the origins, escalation, and transformation of human warfare.

Key Take-aways

  • The characteristics of human warfare are shaped by the coevolution of the following factors:
    • Political economy (or what I call “society type” elsewhere)
    • Political organization (bands/tribes/states/empires etc.)
    • Threat environment (violent competition from other groups)
    • Weaponry
    • Military organization
    • War
  • The pace of change/transformations in warfare and related processes has significantly accelerated three times—first in the late fourth to early third millennium BCE (Middle East), then in the last half of the first millennium BCE (Greece, Rome and China), and again from 1500 to 1945 (Europe).
  • The benefits of war were greater than the relative cost for societies before the Industrial Revolution. The increasing costs have led to a recent decline in warfare.

Important Quotes from Book

Our primary, and admittedly immodest aim in this book is to explain the origins, escalation, and transformation of warfare.1 Central to that aim is describing and explaining how war has coevolved with other factors such as political and military organization, threat environment, political economy, and weaponry. Thus the arc of war is the storyline of war traced over time and space. In elaborating and explaining the arc of war, we make six arguments.

War emerged in different places at different times depending on the presence and absence of critical factors, including the development of hunting/homicidal skills, group segmentation processes, and the interactions among increased organizational complexity, resource scarcity, and conflicts of interest.

War coevolved with other activities, including military and political organization, political economy, threat environment, and weaponry.

Major changes in politico-economic complexity, in particular, have led to occasional transformations in warfare. Weaponry has become more specialized and lethal. Military organizations have expanded. Political organizations have expanded to manage larger and more deadly military forces and more intensified threat environments. The expansion of warfare, however, has not been inexorable. An important constraint is the escalating cost of warfare, which has especially impacted the probability of warfare between industrial states.

The pace of change/transformations in warfare and related processes has significantly accelerated three times—first in the late fourth to early third millennium BCE, then in the last half of the first millennium BCE, and again in the second half of the second millennium CE.

The attempt to centralize regional political-military power is one of the major drivers of periods of acceleration and transformation, especially in the third acceleration, which was concentrated in the western trajectory.

Much of the world did not experience the third acceleration directly (other than as targets), and it remains more agrarian than industrialized. As a consequence, states outside of the western trajectory tend to be weaker, vulnerable to internal warfare, and prone to fight fewer and shorter interstate wars.

In brief, war originated and coevolves with other activities. The pace of evolution and coevolution has been characterized by three periods of acceleration. The contemporary outcomes of these changes and transformations are twofold: (1) strong, industrialized states for which warfare with other strong industrialized states has become very expensive and, therefore, less probable; and (2) weaker, agrarian states that have not experienced the third acceleration in the same way, and that are more likely to engage in internal warfare than in external warfare.

Argument 1: War originated and evolved as a practice over millennia.

Argument 2: War coevolves with other activities, including military and political organization, political economy, threat environment, and weaponry. The basic thesis here is that a substantial change in one of the six spheres is likely to lead to major changes in some or all of the other spheres. A significant increase in external threat might lead to greater political centralization, an expanded military organization, new weapons, and new ways of raising money to pay for larger bureaucracies and armies and for new weapons systems. Similarly, a significant increase in political centralization might also lead to increased tax revenues, an expanded military organization, and new weapons, which contributes to a more threatening threat environment for other actors.

Argument 3: Major changes in political-economic complexity, as manifested in transitions to the predominance of hunting-gathering, agrarian, and industrial production strategies, led ultimately to major changes in warfare.

Argument 4: The evolutionary pace of changes in warfare (and associated processes) has accelerated three times in a revolutionary way.

The first acceleration took place in southern Mesopotamia predominately in the late fourth and early third millennium BCE, as urbanization, population density, and agriculture created new possibilities of scale and kind for intercity warfare. The second acceleration, focused on the eastern Mediterranean and China, occurred in the last half of the first millennium BCE. Near-constant warfare between competitive states created an escalatory spiral in warfare, army sizes, weaponry, and political economies.

The third escalation was centered in European developments in the second half of the second millennium CE. Between roughly 1500 and 1945.

Argument 5: Whereas argument 3 is predicated on evolutionary shifts in the predominant political-economic production strategy, movement within regional evolutionary trajectories is fueled by a competition among states that is characterized by intermittent attempts to centralize political-military power in the region. Successful concentrations of power tend to be temporary and followed by periods of deconcentration of political-military power. The urge to subordinate rivals and to avoid being subordinated by rivals gives political units incentives to upgrade their military organizations and weaponry.

Argument 6: Much of the world did not experience the third acceleration in warfare firsthand. A good proportion of this very large chunk of humanity, not coincidentally, also retains economies that are still more agrarian than industrial. The interaction of these factors implies a different trajectory altogether for the nonindustrial world, which is largely a nonwestern world. States outside the western trajectory have largely agrarian economies. They tend to be weaker and markedly vulnerable to internal warfare, and they fight fewer and shorter interstate wars. Coevolution and the arc of war have worked differently in the global south than in the north.

Rather than simply describe the changes in warfare over time, we construct a relatively simple and overarching theoretical framework. We specify six main factors that account for the evolution of warfare over the last ten millennia: military organization, political organization, weaponry, political economy, threat environment, and war. In the abstract, all six of these factors interact, and there is no single master variable. At various times one or more of the six may be a more significant driver of change than are the others, and any of the six factors retains some potential for bringing about change in the other five. Within this framework, however, we also propose a second, more specific theory: major contextual changes in political- economic complexity—the transitions from hunting-gathering to agrarian and then industrial production—have led to significant changes in military and political organization, weaponry, threat environment, and war. For this particular theory we do privilege one of the six factors as the primary, if highly macro, causal driver.

Although we do not believe that any one factor is always the main driver of change, we will argue that threat environment, political economy, and political organization are the three most important factors, at least most of the time. Shortly we will also stress the very long-term significance of one of the factors, political economy, in accounting for the “arc” of war—its general configuration over at least the past 10,000 years.

Our model posits that:

political economy → political organization → war

Privileging political economy in this fashion provides considerable explanatory power. The predominance of each main type of political economy is associated with a number of variables that have clear bearing on political organization and on warfare.

In general, though, the evolution toward more complex political-economic settings has three effects of particular importance to our analysis. Moving toward greater political-economic complexity generates greater incentives to go to war, at least up to a point. There are all sorts of reasons for going to war but at the core are desires for acquiring more resources and territory or defending what one already has in these areas. If one increases the presence of wealth and the political significance of turf, the incentives for warfare are expanded substantially.

Movement along the political-economic complexity continuum also implies greater resources with which to go to war. More men for armies and navies are available. Weaponry becomes increasingly more lethal. Governments learn how to extend their logistical reach to support military movement. They also learn how to mobilize more financial and labor contributions from their populations for war purposes. More interaction also leads to greater frictions or opportunities to escalate disagreements to full-fledged conflicts. In these three respects—expanded incentives, resources, frictions—the movement toward more of each facilitates the likelihood of more complex political organizations and expanded warfare.

In coevolutionary terms our third theory says that greater political-economic complexity leads to expanded military organizations and increasingly specialized and lethal weaponry. More complex political organizations are needed to manage and survive the expanded warfare and intensified threat environment that result.

The basic idea is that the benefits of warfare often exceeded its costs in the hunting-gathering and agricultural stages. In the industrial stage, however, the costs of war (at least between industrialized states) spiral upward faster than do the benefits.

We can summarize the very long-term processes at work in the following way:

1. Given weaponry, rudimentary military organization, and group identification, war emerged within contexts of scarcity, in response to threats from other groups and/or opportunities to subordinate or exploit other groups.

2. Once the practice of war emerged (and diffused), its escalation in terms of frequency, magnitude, and lethality expanded, depending primarily on the threat environment and increases in political-economic complexity, and secondarily on changes in weaponry and military organization. Macrochanges in the types of predominant production strategies (hunting-gathering, agrarian, and industrial) also hastened the coevolution of warfare and its related factors.

Diffusion is an important factor in explaining the widespread resort to warfare throughout the planet. That is, once warfare emerged here and there, the practice was copied and refined by others who sought to apply the same approach to their own problems.

The core of the second acceleration is best captured by the activities in Greece, Rome, and China. They share at least one background condition, the advent of iron metallurgy, which vastly improved agricultural productivity and consequently led to increases in population sizes. They also share the important common denominator of the intensification of warfare (and the threat environments) between competing states. The outcomes were similar as well in that one state (first Macedonia, then Rome in the west and Qin-Han in the east) defeated their competition and established geographically extended empires at the two ends of Eurasia that survived for a century or two into the first millennium CE.

Weapons became much more lethal, military organizations became larger and more complex, and states became more powerful.

What begins as a change in military technology often affects military doctrine, strategy, and tactics, and sometimes these changes have a broader impact on the nature of military organizations, civil-military relations, the strength of the state relative to society, and political culture. These nonmilitary developments, once underway, have an independent and prolonged impact on the nature of warfare and can also create the conditions under which military innovation is more or less likely.

One way to simplify the changes in military technology and organization of the first millennium BCE is to note that the Assyrians developed their primacy in part on the basis of a combination of infantry, chariot, and siege warfare emphases. Their successors, the Persians, placed greater emphasis on cavalry that had been introduced initially by the Assyrians (courtesy of Cimmerian and Scythian nomads). Alexander the Great merged the eastern emphasis on cavalry with the western emphasis on heavy infantry to become the first European to conquer a goodly proportion of west Asia and northeast Africa. The Romans then took the Macedonian phalanx one step further by breaking heavy infantry units into more flexible, smaller numbers of men, who were less dependent on long spears.

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