This article reviews the recent development of war studies with a focus on the rational choice and the psychological approaches. Scholars in international relations have been studying the origins of war. A traditional theory, Realism, emphasized power as the determinant of international outcomes. However, it did not explain how the distribution of power induces the onset of war. Rationalists connected the missing link by highlighting crisis bargaining as a key process and described the outset of war as a negotiation failure. Wars occur because of the uncertainty problem, the issue indivisibility problem, and the commitment problem. Psychologists applied prospect theory to international conflict behavior and explained how the possible loss of territories drives states to take the risk and escalate crisis bargaining to the onset of war. The latter approach shows more difficulties in reaching a peaceful settlement. Moreover, scholars start using experiments to examine which is more important to explain crisis behavior. Some studies detect the importance of psychological factors in crisis bargaining. We will develop the understanding of conflict behavior by examining the relative importance of the rational choice and the psychological approaches.