During the Cold War, the mission of American forces abroad was to deter the Soviet Union. Today it is to reassure all countries in both regions that there will be no sudden change in the military balance. This is especially important for Germany and Japan. Without the assurance of American protection, they might feel the need for stronger military forces, ultimately including nuclear weapons. The suspicion that the two might adopt such politics would alarm their neighbors, who would feel the need to adopt military policies that would, in turn, alarm Germany and Japan.
American force serves as a barrier to such dangerous chain reactions.
It is therefore important to keep some of them in place. A campaign against nuclear proliferation is a complicated, open-ended task that requires extensive international cooperation. The United States is the indispensable leader of the campaign. A third, principal, post-Cold War purpose of American foreign policy is trade. The passage of the General Agreement on Tariffs and Trade and the North American Free Trade Agreement both expanded the international trading system that has contributed mightily to global economic growth for five decades.