Imagine a gambler willing to bet on conditions a decade in the future. Suppose it is the autumn of 1920, with the Great War of 1914–18 ended in Western Europe just two years ago and fighting still going on in Poland, Russia, and parts of the former Ottoman Empire. In the United States, Republican Warren Harding has just been elected to succeed the Democratic war president, Woodrow Wilson. Our hypothetical gambler is to bet on what will be the posture of the United States in international affairs in the autumn of 1930.

At the moment, the United States is incomparably the strongest power on Earth. Statistics are not kept or reported in today's categories. Gross domestic product will not be measured for another twenty years. Still, in terms of iron and steel production, and energy consumption, the tables on the page opposite show US dominance over the world.

Our hypothetical gambler lacks not only current-style data but also the kind of theoretical writing on international relations that inspires the chapters in this volume. When that theoretical writing does begin to appear, mostly in the United States in the second half of the twentieth century, however, many of the theorists who call themselves realists will cite Thucydides' history of the Peloponnesian Wars as a canonical text. In particular, they will cite the words that he puts in the mouths of Athenian delegates arriving in 416 bce on the tiny island of Melos to demand its complete submission.