Thucydides, the Ancient Greek historian of the fifth century B.C., is not only the father of scientific history, but also of political "realism," the school of thought which posits that interstate relations are based on might rather than right. Through his study of the Peloponnesian War, a destructive war which began in 431 B.C. among Greek city-states, Thucydides observed that the strategic interaction of states followed a discernible and recurrent pattern. According to him, within a given system of states, a certain hierarchy among the states determined the pattern of their relations. Therefore, he claimed that while a change in the hierarchy of weaker states did not ultimatley affect a given system, a disturbance in the order of stronger states would decisively upset the stability of the system. As Thucydides said, the Peloponnesian War was the result of a systematic change, brought about by the increasing power of the Athenian city-state, which tried to exceed the power of the city-state of Sparta. "What made the war inevitable was the growth of Athenian power and the fear which this caused Sparta," Thucydides wrote in order to illustrate the resulting systematic change; that is, "a change in the hierarchy or control of the international political system."
...
The impact of Thucydides' work upon scholars of the Cold War period consists evidence for the relevance of his realist theory in today's world. In fact, while his Peloponnesian War is chronologically distant from the present, Thucydides' influence upon realist scholars in the post-1945 period, and in turn upon American diplomacy, is direct. Specifically, the foundations of American diplomacy during the Cold War with regard to the struggle between the two superpowers and the ethical consequences or problems posed for smaller states caught in the vortex of bipolar competition are derived from his work.
...
Finally, the end of the Cold War requires a re-examination of Thucydidean scholarship and the theories of interstate behaviour which are derived from his work. Furthermore, if there is to be a new world order, the United States must recognize that the dynamics of interstate relations are constantly fluctuating. While there may be certain constants in the behaviour of states and individuals, the possibilities for interaction, cooperation, and conflict are always constant, and often present themselves in new and previously unknown forms. In this case, the study of history is only a guide, not a prescription. If the work of Thucydides is considered in these terms, it will truly be considered a possession for all time, just as the author had intended.