Historians and some scholars of international relations have long argued that historical contingencies play a critical role in the evolution of the international system, but have not explained whether they do so to a greater extent than in other domains or why such differences may exist. The authors address these lacunae by identifying stable differences between war and other policy domains that render the evolution of the international system more subject to chance events than those other domains. The selection environment of international politics has produced tightly integrated organizations (militaries) as the domain's key players to a much greater degree than other policy domains. Because there are few players, no law of large numbers holds, and because militaries are tightly integrated, microshocks can reverberate up to macro-organizational levels. The anarchic character of the international system amplifies the impact of these shocks. The authors explore these phenomena in a range of historical examples.