The authors develop a model of alliances with outside options to study burden sharing in nonbinding alliance agreements. The analysis provides an explanation for the variation in ally contributions to NATO over time and why the post-Cold War period has seen an increase in the use of coalitions of the willing. Additionally, the analysis reveals something of an initiator's disadvantage in burden sharing-the initiator of an alliance action pays a disproportionate cost of the military burden. The authors' argument provides an alternative explanation for why the United States has been consistently the largest contributor to NATO. [PUBLICATION ABSTRACT]