Hans Morgenthau
The German-Jewish emigre who founded modern American realism; his 1948 Politics Among Nations argued that world politics is a permanent struggle for power.
Modern American realism was largely built by a man who had watched a state go mad. Hans Morgenthau was a German-Jewish lawyer who fled the Third Reich, reached the United States in 1937, and by 1948 had produced the book that taught two generations of diplomats and professors how to think about power. Politics Among Nations insisted that world politics is not a morality play but a permanent struggle for advantage, driven by something fixed and unflattering: human nature. People want to dominate; the states they build inherit the appetite. Wish that away, Morgenthau warned, and you get not peace but disaster.
Human nature is the engine
Morgenthau's realism starts one layer below the state, in the human being. He belonged to the tradition running from Thucydides through Machiavelli and Hobbes, which treats the drive for power as a permanent feature of the species rather than a curable defect. Nazi Germany, which had stripped him of his career and nearly his life, was his standing proof that appeals to reason and law do not restrain the ambitious. If people seek power, and people govern states, then states seek power. That syllogism is the whole foundation of classical realism.
Interest defined as power
The second edition of Politics Among Nations, in 1954, opened with what became its most quoted passage: the six principles of political realism. The load-bearing one holds that statecraft is guided by "interest defined in terms of power." A leader's job is not to export virtue but to defend the national interest, and the surest ruin is to confuse your own country's morality with universal moral law. Prudence, not righteousness, is the political virtue. Morgenthau distrusted crusades precisely because a moral certainty that recognizes no limit tends to overreach.
The realist who said no to Vietnam
That distrust made Morgenthau one of the earliest and most prominent critics of the Vietnam War. He had advised the State Department under Kennedy and Johnson; when he argued in public, at a 1965 teach-in and in a televised debate with Johnson's adviser McGeorge Bundy, that Southeast Asia was not a vital American interest and the war was a violation of realist prudence, the administration cut him loose. The episode is the clearest rebuttal to the caricature of realism as warmongering. For Morgenthau, matching commitments to interests could just as easily mean refusing a war as fighting one.
From nature to structure
A generation later Kenneth Waltz kept the realist conclusion but moved the cause. Where Morgenthau located the struggle for power in flawed human nature, Waltz's structural realism located it in the shape of the system itself: with no world government above them, states compete for security whatever their leaders are like. The debate between the two, human appetite versus anarchic structure, still organizes the field. But it was Morgenthau who first made power the subject that serious American thinking about the world could not avoid. He died in 1980, and the discipline he helped found still argues in his vocabulary.