Realism is the dominant theoretical tradition in the study of international relations, and its central claim is austere: states pursue power because they must, not because they wish to. There is no world government. No higher authority enforces agreements, polices borders, or punishes aggression reliably. Each state is, ultimately, alone with its capabilities. In this condition — anarchy, in the technical sense of having no ruler above the states themselves — even peaceful, satisfied states have to act as if their neighbours might one day attack, because the cost of guessing wrong is sometimes existential. Out of that bleak premise the whole edifice is built.
The realist canon runs from Thucydides — whose Melian Dialogue ("the strong do what they can and the weak suffer what they must") is the tradition's founding text — through Machiavelli and Hobbes to the modern theorists Hans Morgenthau, Kenneth Waltz, and John Mearsheimer. Morgenthau's Politics Among Nations (1948) grounded realism in a fixed human will to power; Waltz's Theory of International Politics (1979) relocated the cause to the system itself, arguing that the system's anarchy, not human nature, drives behavior — the move that created structural realism and split it into Waltz's cautious "defensive" variant and Mearsheimer's "offensive" one, in which great powers maximize relative power until they dominate their region. The tradition's strengths are parsimony and predictive grip: it explains why allies eventually quarrel once a common threat fades (NATO's post-1991 drift), why empires expand even when their leaders would prefer not to, why strong states bully weak ones with weary regularity, and why the security dilemma — one state's defensive arming reads as another's threat — recurs across utterly different cultures and centuries. Its weaknesses are also visible: it has trouble with cooperation (which clearly happens, persistently, in trade and arms control), with ideology (the Sino-Soviet split was not just a great-power rivalry but a doctrinal one), with non-state actors (terrorism, multinationals, transnational movements), and with the moral content of foreign policy (some states genuinely behave better than realism predicts). Most working diplomats are realists most of the time and something else some of the time.
The current revival of great-power competition — US-China, Russia-NATO — has been described as the return of realism, after a post-Cold-War interlude in which liberal institutionalist optimism had the upper hand. You can hear the tradition working in the headlines: Mearsheimer's offensive-realist reading of the war in Ukraine — that NATO's eastward expansion predictably provoked a great power guarding its near abroad — became one of the most argued-over claims of the decade, infuriating and persuasive in roughly equal measure. The same logic frames the Taiwan deterrence debate, where capabilities and credibility, not values, are treated as the variables that decide whether Beijing moves. Whether the realists were right all along, or whether the current moment is more historically specific, is the largest active argument in international relations theory.