Mutual optimism as a cause of conflict: Secret alliances and conflict onset

A prominent international-relations theory posits that mutual optimism, due to two sides holding divergent estimates of their relative bargaining power, causes interstate conflict. We develop a theory of mutual optimism in which conflicting bargaining power estimates arise from asymmetric information about which, if any, third parties will join either side in a military dispute. We contend that secret alliances can generate mutual optimism, which increases the probability of conflict. By exploiting secret alliances as a measurable source of private information, we provide the first systematic test of mutual optimism that directly assesses a state's secret capabilities. Optimism exists when a state's secret allies are more numerous or powerful than anticipated by opponents. Our empirical tests-as well as robustness checks-strongly support our theoretical expectation. We conclude that mutual optimism is an empirically, as well as theoretically, important cause of interstate conflict.