'In making alliance commitments to friendly but threatened states, how do states balance the need to credibly deter a potential aggressor while at the same time avoiding the moral hazard of encouraging risky behavior by the ally? When will a state opt for an ambiguous alliance commitment, and how does this strategic ambiguity affect the behaviors of the ally and the threatening state? Benson answers these questions by developing and testing a new theory of alliances and probabilistic commitment. Constructing International Security's substantive importance, theoretical rigor, and empirical sophistication make it required reading for all conflict theorists.' Jack S. Levy, Rutgers University