Treaties governing nuclear-arms reduction and missile defences are languishing for political and more fundamental strategic reasons. START and the ABM Treaty reflect the logic of Mutual Assured Destruction, or MAD, the central organising principle of strategic-arms control during the Cold War. MAD's two basic tenets, as practised by the United States and the Soviet Union, were nuclear overkill and vulnerability to missile attack. Now that the Cold War is over, these tenets no longer command widespread public or congressional support in the United States. A sustainable and bipartisan basis for future US policies geared toward the reduction of nuclear dangers must be built on different ground. While nuclear deterrence will remain important in the twenty-first century, Cold War conceptions of MAD can no longer guide future US decisions regarding nuclear weapons and missile defences. The time has come to replace MAD with a new principle, one that embodies a cooperative approach to reducing nuclear threats.