This paper explains one of the central roles of alliance contracts, the prevention of undesirable military entanglement. The existing literature on alliances argues that entrapment is a major concern for potential and actual alliance partners, but it is difficult to point out clear cases of entrapment. I provide two answers to this puzzle: First, entrapment is a narrower concept than others have realized, and it is rarer than the literature suggests. Second, leaders anticipate entrapment and carefully design alliance agreements before and after states form alliances. I examine the second argument through case studies of US alliance agreements with South Korea, Japan, and Spain.