Abstract : If the United States were subject to a terrorist nuclear attack, its president would face overwhelming political pressure to respond decisively. A well-prepared response could help both to prevent additional attacks and to bring the perpetrators to justice. An instinctive response could be cataclysmically ineffective, inflicting enormous collateral damage without achieving either deterrence or justice. An international security doctrine of Mutually Assured Support can make the response to such attacks more effective as well as less likely—by requiring preparations that reduce the threat. The doctrine requires all subscribing nations to mobilize fully in support of the attacked nation, in return for a promise of nonretaliation. It provides a vehicle for domestic and international leadership, allowing the president to engage the American people, from a position of strength, around an issue that has had little public discussion. The authors describe its rationale, implications, and implementation.