A major paradox in international relations is the widespread fear and anxiety that underlies the security dilemma in times of peace and the prevalence of overconfidence or “false optimism” on the eve of war. A new theory of the causes of war—the Rubicon theory of war—can account for this paradox and explain important historical puzzles. The “Rubicon model of action phases,” which was developed in experimental psychology, describes a significant shift in people's susceptibility to psychological biases before and after making a decision. Prior to making decisions, people tend to maintain a “deliberative” mind-set, weighing the costs, benefits, and risks of different options in a relatively impartial manner. By contrast, after making a decision, people tend to switch into an “implemental” mind-set that triggers a set of powerful psychological biases, including closed-mindedness, biased information processing, cognitive dissonance, self-serving evaluations, the illusion of control, and optimism. Together, these biases lead to significant overconfidence. The Rubicon theory of war applies this model to the realm of international conflict, where implemental mind-sets can narrow the range of bargaining options, promote overambitious war plans, and elevate the probability of war.