The Rubicon Theory of War

halted his army on the banks of the Rubicon River in northern Italy. According to Suetonius, he paused in momentary hesitation, before sweeping across the waters toward Rome with the immortal phrase Alae iacta est (The die has been cast).1 By violating an ancient Roman law forbidding any general to cross the Rubicon with an army, Caesar’s decision made war inevitable. Ever since, “crossing the Rubicon” has come to symbolize a point of no return, when the time for deliberation is over and action is at hand. In this article we set out the Rubicon theory of war. When people believe they have crossed a psychological Rubicon and perceive war to be imminent, they switch from what psychologists call a “deliberative” to an “implemental” mind-set, triggering a number of psychological biases, most notably overconadence.2 These biases can cause an increase in aggressive or risky military planning. Furthermore, if actors believe that war is imminent when it is not in fact certain to occur, the switch to implemental mind-sets can be a causal factor in the outbreak of war, by raising the perceived probability of military victory and encouraging hawkish and provocative policies. The Rubicon theory of war has several important implications for international relations theory and practice. First, it helps to resolve a major paradox in international relations: the widespread fear and anxiety that underlies the security dilemma in times of peace and the prevalence of overconadence on the