Kenneth Waltz
The founder of neorealism, who argued that the anarchic structure of the international system, not human nature, drives states, and that bipolarity is stable.
The most influential idea in modern international relations is that you can explain how states behave without knowing anything about their leaders, their ideologies, or their histories. Kenneth Waltz (1924-2013) built a career on that claim. As the founder of neorealism, also called structural realism, Waltz argued that because the international system has no government above sovereign states, an anarchy, that structure alone shapes what states do, pushing even the most different countries toward the same wary, self-reliant behavior. His 1979 book Theory of International Politics became the most assigned text in American graduate training in the field, the framework that everyone since has either extended or attacked.
Three images of war
Waltz first made his name in 1959 with Man, the State, and War, which sorted every explanation of why wars happen into three "images." The first locates the cause in human nature: people are aggressive. The second blames the internal character of states: some regimes are simply warlike. Waltz's move was to insist on the third image: wars recur because the system is anarchic, with no authority to prevent them, so each state must see to its own security and can never fully trust the rest. The first two images explain why a particular war breaks out; only the third explains why war as such is always possible.
Structure over human nature
Where classical realists like Hans Morgenthau rooted power politics in flawed human nature, Waltz relocated the explanation to the structure of the system itself. States, he argued, are like firms in a market: whatever their internal makeup, competition forces them to act alike or be punished. From this he drew his most contested conclusion, that a bipolar world of two superpowers is more stable than a multipolar one. With only two great powers, each watches the other closely, miscalculation is rarer, and there are no shifting alliances to drag them into someone else's quarrel. It was the counterintuitive argument that made sense of a Cold War that stayed cold, and it is the ancestor of every later structural reading of great-power rivalry, including the Thucydides trap.
More may be better
Waltz followed his own logic to a place most colleagues would not go. If the fear of retaliation keeps great powers cautious, he reasoned, then nuclear weapons are a force for peace, and their slow, controlled spread might even stabilize dangerous rivalries. In a much-debated 1981 essay, "The Spread of Nuclear Weapons: More May Be Better," and a long-running exchange with Scott Sagan, he held that new nuclear states tend to grow careful rather than reckless. He pushed the thesis to its limit in 2012, at age 88, in a Foreign Affairs article bluntly titled "Why Iran Should Get the Bomb," predicting that an Iranian deterrent would calm the Middle East by balancing Israel. Critics found it chilling; Waltz found it merely consistent. Right or wrong, his theory trained a generation to ask a different question: not what leaders want, but what the structure lets them do.