国際危機における単独防衛

―効果とメカニズム―

抄録

An influential conventional wisdom holds that multilateralism is a favorable means of conflict resolution. Likewise, allies’ support in a crisis is usually considered desirable in terms of bringing victory, cost-sharing and legitimacy. This article examines the effect of unilateral defense on crisis escalation and shows that it serves as a tying-hand signal, which can be adopted by a targeted country regardless of its political institutions. To illuminate the mechanism, I provide a formal model of an international crisis where a targeted country chooses whether to unilaterally oppose a revisionist challenger or to acquiesce in short of its allies’ aids. The model demonstrates that unilateral defense enables the target to coerce the challenger to back down. The dishonored alliance ties the target’s hands of burden-sharing and hence signals the target’s strong will and capability of resisting the demand. This is because only a strongly resolved and capable target can accept the cost of future war when there is a positive probability that standing firm leads to the outbreak of war. Therefore, unilateral defense encourages the challenger to acquiesce and consequently prevents the crisis from escalating. Empirical analyses of militarized interstate disputes (MIDs) between 1946 and 2001 support this argument. Unilateral resistance of the target significantly decreases the likelihood of war by three percent, ceteris paribus. The findings are robust across different model specifications including Heckman probit models.