During the period 1816-1975, major powers engaged in approximately 225 serious disputes (situations in which military force was threatened or actually used), but only about 12% of these disputes ended in war (Singer, 1979). What model can explain the governmental decision-making that goes on during disputes and account for the rather small percentage of these situations that end in large scale violence? The purpose of this article is to build and test a computer simulation of basic calculations made by governments during serious disputes, and to predict the individual threats and uses of force that they will undertake, including the ultimate act of going to war. To test the simulation, the actions of both Russia and Japan during a of disputes from 1~~~ to 1904 will be predicted. I begin by discussing the concept and operationalizations of the serious dispute and then turn to a description of the simulation. Finally, the disputes themselves, the modelﾒs predictions, and the fit of these predictions to the observed actions of the Russian and Japanese governments will be considered.