Realism is most influential amongst political scientists, as well as scholars and practitioners of international relations. While realism is a complex and often sophisticated doctrine, its core propositions express a strong suspicion about applying moral concepts, like justice, to the conduct of international affairs. Realists believe that moral concepts should be employed neither as descriptions of, nor as prescriptions for, state behaviour on the international plane. Realists emphasize power and security issues, the need for a state to maximize its expected self-interest and, above all, their view of the international arena as a kind of anarchy, in which the will to power enjoys primacy.

Referring specifically to war, realists believe that it is an inevitable part of an anarchical world system; that it ought to be resorted to only if it makes sense in terms of national self-interest; and that, once war has begun, a state ought to do whatever it can to win. In other words, “all's fair in love and war.” During the grim circumstances of war, “anything goes.” So if adhering to the rules of just war theory, or international law, hinders a state during wartime, it should disregard them and stick steadfastly to its fundamental interests in power, security and economic growth. Prominent classical realists include Thucydides, Machiavelli and Hobbes. Modern realists include Hans Morgenthau, George Kennan, Reinhold Niebuhr and Henry Kissinger, as well as so-called neo-realists, such as Kenneth Waltz.