Why do wars widen? We cannot contain conflicts without first determining why they spread. This study tested three theoretical explanations of war widening: alliances, offense dominance, and the predation and balancing explanation. Alliances and offense dominance both failed to explain why conflicts spread. In addition to theoretical problems, neither the alliance nor the offense dominance argument fits the facts of the four cases examined in this study. The war-widening theory developed in this study far surpassed the other two in explanatory power. Most importantly, it can also be used to predict future trends better than alternative explanations.