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world's stockpile of nuclear weapons has grown to some 50,000 warheads, most of which have greater explosive power than those used on Japan. The use of only a small fraction of today's nuclear weapons would be an unprecedented catastrophe. Over almost the entire postwar period, minimizing the risk of such an event has therefore been a major goal of U.S. policy.

At the same time, the adversarial relationship between the United States and the Soviet Union has led to a U.S. nuclear deterrence policy with the twin, if sometimes conflicting, goals of minimizing the risk of nuclear war and deterring certain classes of conduct by the Soviet Union and, at times, other nations as well. The objectives of deterrence have been the prevention of nuclear attacks on U.S. or allied territories and forces abroad, as well as some kinds of aggression not involving nuclear forces, such as the invasion of Western Europe.

We cannot know the extent to which either nuclear deterrence or the present vast numbers of nuclear weapons have deterred war. The existence of nuclear weapons has certainly added caution to the conduct of U.S.-Soviet relations. The risk of regional conflict escalating into unlimited warfare has constrained the behavior of both countries. Beyond these qualitative observations, we have no way to assess how the number and type of nuclear weapons, their deployment, or the declaratory policy governing their mission have determined the two nations' conduct during the Cold War. The end of the Cold War gives us both incentive and opportunity to examine afresh the objectives of nuclear weapons policy and the mission of nuclear weapons.

The principal U.S. objective remains the prevention of nuclear war, which perhaps alone of all external threats could threaten the continued existence of the United States. Preventing nuclear war means first of all preventing nuclear attack. It also means preventing the occurrence of situations so dangerous that they might lead to the use of nuclear weapons against the United States. In the past, these postulated situations included some massive nonnuclear attacks, and the United States sought to extend nuclear deterrence to prevent them.

Any concept of “extended” deterrence, to deter massive nonnuclear attack on one's own country or its allies, suffers from a basic tension of values. The credibility of a U.S. threat of nuclear retaliation against a nonnuclear attack, however dire its consequences, is impaired if the U.S. homeland would then itself be subject to a nuclear counterstrike (as symbolized by the remark that the United States would not have traded New York for Paris). Therefore, an aggressor planning to initiate a massive nonnuclear attack may or may not be willing to accept the risk of nuclear escalation.

So long as war is possible and nuclear weapons exist, this tension cannot be eliminated; for many years the West decided to live with it. But the

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