Alliances: Why Write Them Down?

States formalize some relations into military alliances. A formal commitment could increase credibility by signaling an intention to come to the aid of another state or by creating commitment by altering the costs and benefits of such intervention. In this review, I lay out three considerations in a decision to intervene in a war. Signals require some costs to trasmit information, and I examine some possible costs in alliances. A state's willingness to intervene could be enhanced by audience costs for failure to honor a commitment. Neorealist arguments about alliances are flawed in asserting that security is a public good and in failing to realize that all states have both status quo and revisionist interests. This review surveys a number of smaller topics in alliances—the tradeoff between arms and allies, burden sharing, alliance management and duration, nonsecurity benefits, and domestic politics.