Classical realism was attacked by other theoretical approaches like neoliberalism, feminism and critical theory. However, the most influential critique came from other realists. Kenneth Waltz became the leading proponent of a new version of realism labeled neorealism (also called structural or systemic realism). His book Theory of International Politics (1979) quickly replaced Morgenthau’s Politics among Nations. According to Waltz, a scientific IR theory must focus on the international system not on the internal characteristics or leaders of states. Waltz developed a systemic theory in which he diverged from classical realism on the causes of state behavior in the system.