The First World War continues to this day to be a source of great controversy among theorists of international relations (IR). Aside from its decisive role in shaping the twentieth century, there is probably one main reason why this particular war, above all others, seems to provoke such heated debate in the IR field: pretty well every major theory of international relations has a dog in this fight. Defensive realists and bargaining theorists, who believe wars occur because of spirals of mistrust and an inability to locate a mutually preferred negotiated peace, consider the First World War to be the preeminent case of actors falling into a war they would otherwise have wanted to avoid. Liberals and constructivists, who see wars arising from unit-level pathologies in the heads of leaders and their societies, can look to 1914 as a case where hypernationalism, class warfare, cartel politics, and psychological distortions led actors to race to the front in the hope of satisfying unmet domestic and personal needs. Structural neorealists, who argue that multipolar systems force great powers to support critical allies in a crisis, even at the risk of total war, view the First World War as their main example of such chain-gaining dynamics.

This chapter challenges each of these broad approaches to explaining the First World War. Consistent with dynamic versions of systemic realism founded upon the logic of preventive war, I argue that the war that broke out in that fateful summer of 1914 had one, and only one, primary cause: the profound fear among German leaders that their nation was in irreversible decline relative to a rising Russian colossus. For the key decision-makers in Berlin, all other apparent “causes” were either not important or only secondary to this preventive logic. Elsewhere, I have laid out a new approach to thinking about preventive war, one that differentiates between the effects of decline across bipolar and multipolar systems, and between decline that is inevitable versus temporary and reversible.

When it comes to the study of regional conflicts, realists as a group have been loath to put forward a clear and generalizable theory. This is not to say that realists have not offered insights into the probability of militarized conflict in particular regions. In the early 1990s, for example, John Mearsheimer and Aaron Friedberg offered well-known and controversial arguments for the likelihood of war in Europe and the Far East based largely on the insights of realist logic. Yet there are few realist scholars who have attempted to derive a theory of conflict that would predict the relative stability of different regions based on an understanding of the power dynamics within these regions, as opposed to simply examining how external great powers might exploit regional politics for their own purposes.

This chapter will sketch the initial outlines of a realist theory that does seek to explain and predict the relative levels of stability across regions, both in historical terms and within the contemporary global system. Drawing on my previous work on major war between great powers, I will attempt to show that the logic of dynamic differentials theory (DDT), a structural realist theory designed to explain major wars at the level of the global great power system, can also be used effectively, with some qualifications, at the level of regional subsystems. Dynamic differentials theory argues that crises and wars are most likely when the most dominant military states in a system begin to anticipate steep and largely inevitable decline and thus start to fear the future intentions of rising actors. As I will discuss, whether these dominant states will initiate preventive attacks or destabilizing crises that might avert their decline will depend on a host of important structural variables, including the polarity of the system (in a regional subsystem, the number of important actors), the offense–defense balance, and the levels of relative economic and potential power. For now, it is important to grasp the essence of the approach: wars and militarized conflicts are driven by the dynamics of the power balance, and profound decline by the leading military state or its nearest rival can destabilize a system to the point of bilateral or subsystem-wide war.