Realism helps us see how the lack of hierarchy in authority at the systemic level creates rules that confine the choices available to states. Similarly, the emphasis on power explains why some states are more successful in achieving their goals than are others. This approach to world politics called realism has a long, distinguished history and offers a coherent, parsimonious explanation for much of what goes on across the globe.
Realism theory is different from liberalism and postinternationalism. Liberalism promotes the freedom to pursue economic gain, liberty to participate in the affairs of public life, respect for political human rights, and minimal government. Postinternationalism comes from the presumption that accelerating change and deepening complexity are the major tendencies at work in the world. Realism, liberalism, and postinternational polaics paradigms have some common elements but they also rest on different and contradictory, premises.
Hans Morgenthau (1948) first expounded a theory on international relations which explained the past and current events and which will be the likely direction and shape of future relations. Morgenthau's ideas is similar to writings on world politics and represented continuity with the past. The realist approach stems from Thucydides, the chronicler of the ancient Peloponnesian War, who wrote, "The strong do what they have the power to do, the weak accept what they have to accept." Thucydides pushed for a first-class navy and the wealth of empire gave an edge to Athens. (Thucydides, 1978). It was "the growth of Athenian power and the fear this caused in Sparta" that, in Thucydides' opinion, caused the war. The fact of Athenian power and the fact, known to his readers, that Athens ultimately lost the war creates a terrible tension in his book. (Robert Connor, 1984). Athens, a powerful state lost the war because it overextended itself and fell victim to its own sense of grandeur. Its citizens forgot the necessity for moderation and denigrated the virtues of taking justice as well as advantage into their political calculations. Power, as expressed in ships and money, and the moral character of the warring cities accounted for the final outcome of the war.
Power holds the two strands of realism that have evolved in the modern era. Traditional realism, which evolved in the 1930s and in the post-World War II period, is a form of realism grounded in a view of human nature. It points out that humans are self-interested, rational, and seek power; qualities that lead to the consistent, regular behavior of states. Traditional realism holds that we live in "a world of opposing interests and of conflict among them, moral principles can never be fully realized, but must at best be approximated through the ever temporary balancing of interest and the very precarious settlement of disputes." (Buzan, et.al., 1984).
Neorealism or structural realism, highlights the structure of the international system rather than human nature to account for the behavior of states. Individual preference does not particularly count, since the individuals themselves do not matter a great deal in explaining the behavior of states. This theory proposes that the lack of central authority in the international system causes states to behave the way they do. The set-up of the international system forces states to attend not just to their own interests but to any changes in the power of other states.