The ‘‘second nuclear age’’ is a loose term that has been used to describe the spread of nuclear weapons following the end of the Cold War. The phrase is rarely used with precision or consistency. Sometimes it is intended to emphasize the ‘‘new’’ problems of nuclear proliferation. But cursory reflection often shows that many of these problems, such as deterrence, are not really new at all. They arose in the ‘‘first nuclear age’’ of the Cold War, and far earlier in history.

Defining more precisely the ‘‘second nuclear age’’ permits one to make a critical point: the nuclear age that began in 1945 is not a uniform structure. Rather, it comprises two very different divisions. Our cognitive map of national security and international order was overwhelmingly formed in the first nuclear age, and it is applied unthinkingly to the second. Without recognizing it, legacy concepts (such as deterrence), distinctions, and strategies treat the two eras the same, overlooking the basic differences between them.

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