Realism is the oldest and, among diplomats and soldiers, perhaps still the most influential way of thinking about world politics. Realists see themselves as describing the world as it is rather than as we might wish it to be. Their best defenders insist this is not cynicism but responsibility: leaders who ignore power endanger the very people they are meant to protect.

Ancient and early modern roots

Realists often claim the Athenian historian Thucydides as their founder. His history of the Peloponnesian War, fought between Athens and Sparta from 431 to 404 BCE, gave a famous explanation of its cause, in Richard Crawley's 1874 translation: "The growth of the power of Athens, and the alarm which this inspired in Lacedaemon, made war inevitable." Thucydides explained the war by Athens' growing power and the fear it caused in Sparta, not by the quarrels that set it off. Later writers added to the tradition. Niccolò Machiavelli advised rulers to judge actions by their results for the state. Thomas Hobbes, in Leviathan (1651), argued that people living without a common power over them are in "such a war as is of every man against every man."

Classical realism

After the failure of the League of Nations, the British historian E. H. Carr wrote The Twenty Years' Crisis (1939). Carr attacked the belief that law and public opinion alone could keep the peace, a belief he called utopian. Hans Morgenthau, a German Jewish refugee who taught in Chicago, systematized realism in Politics Among Nations (1948). Classical realists such as Morgenthau rooted the struggle for power partly in human nature, in a lasting human drive to dominate. Statesmen, he argued, should think in terms of the national interest understood as power. For Morgenthau, prudence, the careful weighing of consequences, was the supreme political virtue. He also warned that crusading moralism in foreign policy can be more dangerous than open self-interest, because a nation sure of its own righteousness recognizes no limits.

Structural realism

Kenneth Waltz's Theory of International Politics (1979) moved the cause from human nature to the system. Neorealism, or structural realism, argues that anarchy forces states to rely on self-help for their security: since no one is obliged to rescue them, they must look after themselves. What differs from one system to another is the distribution of capabilities, meaning how power is spread among the leading states. For Waltz, states are mainly defensive, aiming to survive rather than to dominate.

John Mearsheimer's The Tragedy of Great Power Politics (2001) offered a harsher version. Offensive realism holds that great powers seek to maximize their relative power, ideally becoming the dominant power in their own region, because more power is the surest guarantee of survival.

The security dilemma

One of realism's most useful ideas explains how states that want only safety can still end up in conflict. The term was coined by the German American scholar John Herz in 1950. The security dilemma is the situation in which steps one state takes to make itself safer make others feel less safe, so they respond in kind and everyone ends up more insecure than before. A navy built for defense looks like a threat to a neighbor, which builds its own. Nobody needs to be wicked for an arms race to begin.

Strengths and criticisms

Realism's defenders point out that it has often counseled restraint. Morgenthau opposed the Vietnam War, and in 2002 a group of prominent realist scholars publicly argued against invading Iraq. They judge foreign policy by results, not intentions.

Critics reply on several fronts. A common criticism is that realism struggled to foresee or explain the peaceful end of the Cold War, when a superpower gave up its empire in Eastern Europe without a fight. Critics also note that states cooperate far more than realism predicts, through trade agreements and institutions, and that realism treats states as single units, ignoring the domestic politics that liberals stress. Finally, some ethicists worry that treating morality as secondary can excuse cruelty, a worry the next example sharpens.

Worked example: the Melian dialogue

In 416 BCE Athens sent a fleet to the island of Melos, a Spartan colony that wanted to stay neutral. Thucydides wrote the negotiation as a dialogue. The Athenians refused to discuss justice, telling the Melians, in Crawley's translation, that "the strong do what they can and the weak suffer what they must." The Melians appealed to fairness, to the gods and to the hope of help from Sparta. The Athenians answered that hope was a comfort for those who could afford to lose. Melos refused to submit. After a siege, Athens killed the men of military age and enslaved the women and children.

Realists read the dialogue as a lesson in the logic of power: the Melians misjudged their position and their allies, and paid dearly. But Thucydides placed the episode just before Athens' disastrous expedition to Sicily, and many readers think he meant it as a warning about the arrogance of power, not a celebration of it. Both readings are possible, which is part of why the text is still argued over. For a course grounded in equal human worth, the case raises a question realism cannot answer alone: whether survival justifies anything, and whose survival we mean when we say that "the state" survived.