Since the 1990s, in the wake of humanitarian emergencies, violent civil wars, and terrorist attacks, the tension between the norm of state sovereignty and the need to prevent atrocities has become more intense, forcing the UN and great powers to repeatedly ponder what circumstances, if any, justify international intervention. In this magisterial study, Doyle provides the most thoughtful and searching exploration yet of this dilemma. The book builds on John Stuart Mill’s classic 1859 essay on the norm of nonintervention and the prudential terms for its violation. Doyle finds Mills’ analysis flawed but sufficiently compelling to use as a starting point for the construction of a moral logic for liberal interventionism. He sifts through a rich array of cases from the nineteenth century to the present day to ascertain the costs and consequences of intervention, identifying circumstances that justify exceptions to and overrides of the nonintervention principle. Affirming the progress represented by the UN's “responsibility to protect” doctrine, which was formally articulated in 2005, Doyle makes the case for limited, cautious, and multilateral interventions that require both a license and a leash.