Game theory is a mathematical model to explain choices and behavior in strategic situations, based on the assumption of rationality. In International relations, game theory is used in order to explain the behavior of two or more actors in terms of initiating a conflict or more broadly, in solving a dispute. The rational assumption is that conflicts are costly, and thus, there is always a better way for both parties involved (that is, if the two actors participating in war would have known their actual strength, and thus the actual outcome of war, they would have settled beforehand on that outcome, and not spend millions of dollars and many lives to achieve that). Here is where game theory comes in - trying to explain how, instead of reaching an agreement, war starts, proposing lots of models, such as spiral escalation, signaling and information assimetry or failure. J. Fearon wrote a lot on this, look his 1995 article for an introduction.