The dissertation explores the phenomenon of joining behavior--non-neutral interventions by third party states in interstate conflicts. The opportunity and willingness theoretical framework (Most and Starr 1989) is used to develop a model of third party intervention that integrates simultaneously intervention decision, alignment choices, and selection of specific intervention techniques. Within the general model of third party intervention, two models of third party's preference formation--a rational choice and a homophily-based model--are compared. The models are empirically tested with newly collected data on interventions in interstate disputes for the 1946-2001 period. The data expand current knowledge on third states' activities by including information on non-military--diplomatic and economic--intervention techniques. Opportunity factors are found to predict effectively third parties' intervention; while willingness shapes alignment decisions and selection of intervention techniques. Strategic and homophily-based similarities with the state supported in a conflict and dissimilarities with the state being antagonized are found to matter equally in shaping third parties' decisions. Methodologically, this study addresses a variety of selection issues present in current research on joining behavior. Theoretically, it speaks to a variety of international relations issues, such as balance-of-power and bandwagoning, spatial diffusion of conflict, foreign policy substitutability and decision-making, and alliance formation and reliability
