The tradition that analyzes politics in terms of power, interest and the observed behavior of people and states rather than in terms of how they ought to behave. Realism is a claim about method before it is a claim about policy: that political understanding begins with what is, and that moralism about what ought to be tends to obscure rather than illuminate.
Thucydides is its usual ancient ancestor and the Melian Dialogue its founding text. Machiavelli made it modern, arguing in The Prince that a ruler who acts as goodness requires among people who are not good will be destroyed, and that political conduct must be judged by results. Hobbes systematized the psychology — the state of nature as a condition of mutual fear from which sovereign power is the only exit. Weber gave it its most careful moral statement in the distinction between an ethic of conviction and an ethic of responsibility, holding that political actors answer for consequences rather than intentions.
In the twentieth century realism became the dominant framework in international relations. E. H. Carr's The Twenty Years' Crisis (1939) attacked interwar idealism as the self-interest of satisfied powers dressed as universal principle. Morgenthau grounded state behavior in a permanent drive for power. Waltz relocated the explanation from human nature to the anarchic structure of the state system itself, arguing that states behave similarly under similar constraints regardless of their internal character. Niebuhr supplied a theological version, holding that collective moral behavior is systematically worse than individual.
The standing objection is that realism smuggles in the values it claims to bracket, and that describing power politics as permanent helps make it so. Realists generally answer that the record of foreign policies built on the denial of interest is not encouraging — an exchange that has repeated, without resolution, at every major turn in the tradition's history.