The two key questions in national security decision-making and international relations theory remain: When do states make war and engage in conflict? When do states make peace and engage in cooperation? Despite the importance of these questions, the “why” of decision-making continues to be debated by foreign policy practitioners and scholars. The ability to predict, even probabilistically, the outbreak of war and the probability of peace will provide an important tool for policy makers, and a starting point for future research. This book, drawing from the author’s experience as a senior military leader, planner, and diplomat; along with recent work in cognitive psychology, provides a new approach to simulating the process of decision and predicting the likely results. Using a computer simulation of how leaders process and interpret information, it shows how the interaction between the situation and other leaders’ decisions can cause both perceptions and preferences to rapidly shift – leading quickly from calm to crisis and back. Foreign policy practitioners, national security professionals, and scholars will find this new approach to be useful in today’s challenging environment.