We build up a general purpose decision model to predict the choice between going to war and staying at peace for a rational decision-maker. This model articulates root causes such as the risk of future war and parameters such as potential gains in case of victory, potential losses in case of defeat, the probability of victory and the war human losses. We apply and calibrate this model to the case of German and French decision-makers at the very end of July 1914, taking into account the decisions already taken by Austria-Hungary and Russia and the uncertainty surrounding the decision of Great Britain. We assume a short war that does not last beyond 1914. Our model predicts the entry into the war of Germany and France, the argument of preventive war (going to war today rather than tomorrow) proving to be decisive for both countries, with the added benefit for France of the potential recovery of Alsace-Moselle in the event of victory. The computation reveals that of the two countries, it was France that seems to have the most interest in the war, making it possible to explain the passive behavior of the French leaders, Raymond Poincaré in the first place, who, if they did not provoke the war, did not really try to avoid it either. JEL Codes: N44, D74, D81.