Approval from the United Nations or NATO appears to have become a necessary condition for US humanitarian military intervention. Conventional explanations emphasizing the pull of legitimacy norms cannot fully account for this given that US policymakers vary considerably in their attachment to multilateralism. This article argues that America’s military leaders, who are consistently skeptical about humanitarian intervention and tend to emphasize its costs, play a central role in making multilateral approval necessary. As long as the top-ranking generals express strong reservations about intervention and no clear threat to US national security exists, they can veto the use of force. In such circumstances, even heavyweight “humanitarian hawks” among the civilian leadership, who initially may have wanted to bypass multilateral bodies to maximize US freedom of action, can be expected to recognize the need for UN or NATO approval—if only as a means of mollifying the generals by reassuring them about the prospect of sustained multilateral burden sharing. Two case studies drawing on interviews with senior civilian and military officials illustrate and probe the plausibility of the argument.