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.