Sagan and Waltz have taken two recent, divergent articles and added a pair of rebuttals. The result is a short and worthwhile but inconclusive debate about whether the spread of nuclear weapons is a good thing. Waltz, one of the most influential theorists of international relations, expresses a degree of equanimity about the consequences of nuclear proliferation that most members of the foreign policy establishment will find horrifyingly complacent. Sagan, considerably more junior but widely published on the organization of nuclear strategy, powerfully argues the dangers of preventive war, accident, and miscalculation. Although both fall back on historical examples, one is struck by the degree to which history may not provide a very clear guide about the risks (or, if one agrees with Waltz, the rewards) to an international political order arising from nuclear proliferation.