Unpacking Alliances: Deterrent and Compel lent Alliances and Their Relationship with Conflict, 1816-2000

Do alliances deter aggression? I develop a typology of deterrent and compellent military alliances to better define the possible conditions for intervention and use new data from 1816 to 2000 to analyze the relationship between alliances and conflict. First, unconditional compellent alliances, a category that represents a minority of alliances, are associated with a 249% increase in the likelihood of conflict when the prospective initiator is an alliance member. Such alliances are especially associated with violent conflict. Second, conditional compellent alliances exhibit no discernible relationship with conflict. Third, deterrent alliances contingent upon the adversary's attack are associated with an 18% decrease in the likelihood a third party will initiate a conflict with an alliance member. Minor powers holding such alliances with major powers are unlikely to be attacked violently. Fourth, other categories of deterrent alliances do not deter violent conflict. In fact, deterrent alliances that permit preemptive defense can increase violent conflict.