It was Theodore Roosevelt and his circle who first put forward the idea that because the United States had grown to maturity as a power, it ought now to play a commensurate role in world affairs. By 1898 America's world role was no longer a theory, it was a fact. In that year American forces defeated Spain and annexed Cuba and the Philippines and also incorporated Hawaii as a territory. Over the next decade and a half the United States also became deeply embroiled in the affairs of Mexico, Latin America, and the Orient. Yet when the First World War broke out in 1914 a majority of Americans still did not see why the United States need become involved. It was three years before Woodrow Wilson, taking the nation to war against Germany, acknowledged that the United States was not only a world power but potentially the strongest of all the powers by virtue of population, resources, and industrial development.

It is the thesis of this essay that for a hundred years, ever since the Spanish-American War, the power of the United States and its involvement in the outside world have been growing rather steadily, while American attitudes toward that evolution have oscillated wildly. Seen from the outside, in other words, as a reality that other nations have had to confront, the growth of American power has been rather constant; seen from the inside, as an element of domestic politics and of national psychology, American involvement in the outside world has fluctuated in a process of action and reaction, systole and diastole.