Systemic theories of international politics rarely predict conflict short of cataclysmic systemic wars, and dyadic theories of conflict lack systemic perspective. This article attempts to bridge the gap by introducing a two-step theory of conflict among Great Powers. In the first stage, states engage in a dynamic, ongoing process of managing the international system, which inevitably produces tensions among them. In the second stage, relative levels of security-related activity determine how and when those tensions erupt into disputes. A test of the theory on Great Power conflicts from the nineteenth century supports the argument and, moreover, favors the deterrence model over the spiral model as a proximate explanation of conflict in the second stage.