  In 1979 Waltz wrote that in the nuclear age the international bipolar system, based on two superpowers—the United States and the Soviet Union—was not only stable but likely to persist (176–7). With the fall of the Berlin Wall and the subsequent disintegration of the USSR this prediction was proven wrong. The bipolar world turned out to have been more precarious than most realist analysts had supposed. Its end opened new possibilities and challenges related to globalization. This has led many critics to argue that neorealism, like classical realism, cannot adequately account for changes in world politics.