While the United States staked claims around the globe, a war of unprecedented scale was brewing in Europe. The military buildup of Germany, a rising power, terrified its neighbors. To the east, the disintegrating Ottoman Empire was losing its grip on the Balkans. Out of these conflicts, two rival power blocs emerged: the Triple Alliance (Germany, Austria-Hungary, and Italy) and Triple Entente (Britain, France, and Russia). Within each alliance, national governments pursued their own interests but were bound to one another by both public and secret treaties.

Americans had no obvious stake in these developments. In 1905, when Germany suddenly challenged French control of Morocco, Theodore Roosevelt arranged an international conference to defuse the crisis. Germany got a few concessions, but France — with British backing — retained Morocco. Accomplished in the same year that Roosevelt brokered peace between Russia and Japan, the conference seemed another diplomatic triumph. One U.S. official boasted that America had kept peace by “the power of our detachment.” It was not to last.