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ently requires that assurance of reliable control be conveyed as credibly as the threat of retaliation. If one side ever interpreted the alerting procedures the other undertook as a protective precaution as decisive evidence of an impending attack, an unintended war might be triggered, with consequences well beyond any other imaginable human disaster.

The United States and the Soviet Union have devised weapons design principles and organizational procedures to assure both themselves and their opponents that deployed nuclear weapons will not operate unless instructed to do so. They have also deployed multiple warning systems designed to preclude any misjudgment by central political authorities. These provisions are a necessary complement to the many measures taken to assure that strategic forces will retaliate successfully if attacked. They have been successful for over 40 years in avoiding any actual or imminent explosion of a nuclear weapon. Assisted by an overriding political judgment that war is not in fact imminent despite the preparations that make it thoroughly possible, the warning systems have recorded and corrected numerous false indications of attack without ever approaching a catastrophic misjudgment or a breakdown of control. But the command structures of U.S. and Soviet strategic forces have never experienced the pressures of an intense crisis involving the simultaneous alerting of forces. While reassuring, experience to date cannot give definitive evidence about what would happen if a severe crisis altered operational routines and normal political judgments.

With accustomed patterns of evidence and inference disrupted, current warning systems could encounter serious difficulty in proving that an attack was not underway when the prevailing judgment began to suspect that it was. 1 The probability of that happening is presumably very low, but it can only be surmised, not demonstrated. This uncertainty and the enormous potential consequences argue that measures should be taken to strengthen reassurance as the problem of deterrence becomes less demanding.

In our view, four steps could help remedy this situation. The first is a continued program to improve the protection of the command and control system even while the forces are being reduced. The second involves fitting naval weapons with the devices commonly known as permissive action links (PALs) that have been used on other components of U.S. strategic force to physically embody authoritative political control. When installed, PALs make enabling the weapon or gaining access to the warhead itself dependent on receiving a special code from a higher command. The third involves cooperative measures that could increase the ability, if doubts ever arose, to prove that a strategic attack was not under way. The fourth involves a review of targeting practices to ensure that reductions in numbers of weapons systems do not lead to any questions about coverage of essential targets or to pressures for rapid reaction.

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