In the field of international relations, realism has long been a dominant theory, from ancient military theories and writings of Chinese and Greek thinkers, Sun Tzu and Thucydides being two of the more notable, to Hobbes, Machiavelli and Rousseau. It is the foundation of contemporary international security studies. The twentieth century classical realism is mainly derived from Edward Hallett Carr's book "The Twenty Years' Crisis". The realist views anarchy and the absence of a power to regulate the interactions between states as the distinctive characteristics of international politics. Because of anarchy, or a constant state of antagonism, the international system differs from the domestic system. Realism has a variety of sub-schools whose lines of thought are based on three core assumptions: groupism, egoism, and power-centrism. According to classical realists, bad things happen because the people who make foreign policy are sometimes bad. Beginning in the 1960s, with increasing criticism of realism, Kenneth Waltz tried to revive the traditional realist theory by translating some core realist ideas into a deductive, top-down theoretical framework that eventually came to be called neorealism. "Theory of International Politics" brought together and clarified many earlier realist ideas about how the features of the overall system of states affects the way states interact:  The main theories of neorealism are balance of power theory, balance of threat theory, security dilemma theory, offense-defense theory, hegemonic stability theory and power transition theory. Liberalism has a shorter history than realism but has been a prominent theory since World War I. It is a concept with a variety of meanings. Liberal thinking dates back to philosophers such as Thomas Paine and Immanuel Kant, who argued that republican constitutions produce peace. Kant's concept of Perpetual Peace is arguably seen as the starting point of contemporary liberal thought. Economic liberalism assumes that economic openness and interdependence between countries makes them more peaceful than countries who are isolated. Eric Gartzke has written that economic freedom is 50 times more effective than democracy in creating peace. Globalization has been important to economic liberalism. Liberal institutionalism views international institutions as the main factor to avoid conflicts between nations. Liberal institutionalists argue that; although the anarchic system presupposed by realists cannot be made to disappear by institutions; the international environment that is constructed can influence the behavior of states within the system. Varieties of international governmental organizations (IGOs) and international non-governmental organizations (INGOs) are seen as contributors to world peace.