During the Cold War, it was assumed that stable deterrence would work because both sides were "rational opponents." The current debate over missile defense systems has become more complex as we consider irrational opponents — for instance, rogue states that have gotten possession of Russian nuclear weapons. In addition, deterrence becomes irrelevant against terrorists when we do not know whom to threaten or punish. The author believes that no clear-cut case can be made for or against the anti-ballistic missile (ABM) and national missile defense. Although there are some advantages to a missile defense system (if it works at all), the system cannot protect against all potential threats, and it will incur high costs, both political and monetary. He believes that the costs of a national missile defense system, particularly the real budgetary costs, outweigh the advantages. But, he concludes, setting the balance between the potential advantages and potential costs of an American ABM system, and between a thin and a robust national missile defense, is properly a political decision.