After many centuries in which war between the leading powers in the system has been a defining characteristic of international relations and perhaps the primary determinant of change in the international system, we have witnessed five or six decades without a single war between the great powers - the longest period of great power peace in at least six centuries. The great powers continue to build up their armaments, they intervene in weaker states, and they engage in protracted counterinsurgency warfare, but almost no one expects a war between the leading powers in the system for the foreseeable future. Wars between weaker states have also been declining in frequency, although not quite to the same extent or with the same degree of optimism among observers about future trends, as evidenced by current worries about an Israeli-Iranian war. The one form of warfare that has not declined over the last half century is civil war, which increased sharply in frequency after the mid-1950s before pulling back, though somewhat erratically, after the late 1980s.
These patterns constitute an interesting puzzle, and they have generated considerable debate about the future of warfare. Will the peace among the great powers continue? Will interstate wars among weaker states continue to decline in frequency? Will civil wars follow a similar pattern, with the more conflict-prone states becoming more and more like the least conflict-prone states? Why are some types of warfare decreasing in frequency while other types of warfare continue to persist and even increase in frequency? Why has the European region, long the cockpit of major power warfare, become markedly pacific while other regions are much more conflict prone? Is there a relationship between the frequency and severity of warfare, with wars becoming less frequent as they increase in lethality?
Rather than addressing these questions individually, in The Arc of War we attempt to construct a single integrated theoretical framework that can provide logically coherent answers to all of these questions and explain the different patterns involving different types of warfare. At the same time, we anchor our theoretical generalizations in the history of warfare as it has evolved from its origins nearly ten millennia ago to the present day. Among the questions we ask are when, and under what conditions, did human warfare begin? How did warfare subsequently evolve? What causal mechanisms have driven these processes? Has the evolution of warfare been uniform and linear, or has it followed different trajectories under different conditions? We argue that the same general conditions and processes that led hunter-gatherer groups to develop into rudimentary political units and to engage in sustained, organized violence against each other have shaped the evolution of warfare during the past ten millennia.
That evolution, however, has been anything but a linear escalation to ever more lethality and destruction. It has proceeded in fits and starts, occasionally accelerating while at other times even receding in activity. Different regions, moreover, have experienced distinctly different trajectories, both because of different threat environments and different ecological conditions. For instance, tropical heat and limited metallurgy discouraged the wearing of metal armor in Central America while nomadic raiders in Eurasia encouraged the development of heavily armored knights and the breeding of larger horses to carry them in various parts of Eurasia. To take a different example, Europe at the dawn of the modern age five centuries ago was characterized by a highly competitive threat environment. This led to larger armies, to the development of military organizations and more lethal weaponry, to the extraction of more and more societal resources to maintain those armies, and to the centralization of political power. These processes only increased the threat environments of other states, leading to further escalation and frequent wars.
We find similar patterns throughout history. The nature of the threat environment, political and military organization, the political economy, weaponry, and war co-evolve, in the sense that a significant change in one or more of the factors induces changes in the other factors. The arc of war has no single driver, and it has varied during hunter-gatherer, agricultural, and industrial periods of political economy, but we argue that changes in the political economy, such as the revolutions in agrarian and industrial production, have been first among equals in generating co-evolutionary changes.
This conceptual framework guides our examination the origins of war and its escalation through three major accelerations - in southern Mesopotamia predominately in the late fourth and early third millennium BCE, in the eastern Mediterranean and China in the last half of the first millennium BCE, and in the modern system centered in Europe from roughly 1500 to 1945. We use the same framework to analyze the evolving patterns of the last six decades, when warfare between the great powers has vanished and when civil wars within the weakest states in the system have been increasing in frequency for much of the period.
One of our main arguments is that warfare between highly industrialized states has become increasingly costly - both to prepare for and to participate in. As a consequence, warfare between these types of states has become less probable. The same argument, however, does not apply to warfare between highly industrialized states and states with predominately agrarian economies. Intervention by powerful states into the affairs of weaker states should be expected to continue. Yet one of the byproducts of intensive warfare in Europe (1494-1945) was the development of relatively strong states among the political organizations that survived this escalating bloodshed to total war.
To survive the march toward total warfare, states, their decision makers, and their populations had to learn new ways to mobilize resources to engage in intensive combat. One result was that some states expanded their capacities and their linkages with their citizenry. States that have not participated in this process have lower capacity and legitimacy; they provide fewer services to their populations; and they command less loyalty. They are more likely to be challenged internally, less capable of dealing with those challenges, and more likely to experience major insurgencies and civil wars.
We see that the complex patterns of warfare in the contemporary era, as well as those in many earlier eras, can be explained by a single theoretical framework that highlights the co-evolution of changes in political economy, military and political organizations, threat environments, and weaponry. Warfare is not becoming extinct, but certain types of warfare have become less probable while other types have become more likely over the course of the last 50 years and the last 500 years. The evolution of war is neither linear over time nor uniform for all parts of the world. But it is not mysterious either. The Arc of War provides a useful way of conceptually packaging and summarizing the origins, escalation, and transformation of warfare over nearly ten millennia.