Blainey (1988) argued that crises are more likely to end in war when two nations disagree about their relative power. Fey and Ramsay (2007) claim that this widely used “mutual optimism” explanation is theoretically incoherent. Their criticism neglects the need to specify a behavioral causal mechanism that links beliefs to the outbreak of war. We show how the rationalist game-theoretic work on the causes of war provides such mechanisms—the risk-return trade-off and costly signaling—and demonstrate that these models are immune to Fey and Ramsay's critiques. We also show that the class of models Fey and Ramsay propose make the substantively unwarranted assumption that an actor can unilaterally impose peace on an opponent who strictly prefers war. Their finding that war does not occur in equilibrium has nothing to do with mutual optimism. We conclude that the mutual optimism explanation can be grounded on firm rationalist foundations.