For most of its history American foreign policy has been episodic. In the first century and a half of independence, the governing principle was to stay out of foreign affairs altogether. The United States may have chased down Barbary pirates, huffily engaged in border disputes and intervened in one Caribbean island or another, but even external wars were comparatively small beer – whether it was thumping the Mexicans in the 1840s or humiliating the Spanish in 1898. Instead, domestic considerations were everything: expanding westward, the Civil War, getting rich, building the new nation.

Foreign policy, such as it was, existed without any noticeable framework or sense of strategy. The Monroe Doctrine in 1823 warned European states to stop meddling in the affairs of the western hemisphere, but this was largely declaratory and counted on the indulgence of the British navy for enforcement. Theodore Roosevelt conducted a personal foreign policy of muscle-flexing, but to no obvious end. To justify America’s belated entry into the First World War, Woodrow Wilson proclaimed a series of international principles, all of which evaporated from the American mind shortly after the fighting ended.

Things changed fundamentally in the 1940s. Following the century’s most brutal war, America developed a global foreign policy and under Presidents Truman and Eisenhower created a national security infrastructure to give it substance. The central focus of American policy during the Cold War was the containment of the Soviet Union