World War I pitted the Allies (Great Britain, Russia, France, and, later, Italy) against the Central Powers (Germany and Austria-Hungary). In August 1914, Wilson proclaimed U.S. neutrality, and urged the public to remain neutral in opinion as well. The American public, however, was partial to the Allies: though most Americans were glad to be remote from the war, strong emotional, historic, and economic ties to Great Britain and France meant great public sympathy for the Allied cause. While American investment in the Central Powers nations dwindled between 1914 and 1917, it surged in the Allied nations. American sources provided weapons, food, and funding to the Allies equal to nearly one hundred times that provided to the Central Powers. Wilson himself seemed to favor an Allied victory, in part because he saw a victory by Germany and its autocratic ruler, Kaiser Wilhelm, as antagonistic to his vision of a world order based on liberalism, democracy, and capitalism. Nonetheless, he clung to neutrality.

After 1914, it became increasingly clear that American neutrality would be difficult to maintain. British naval vessels seized American ships headed for German ports and filled the North Sea with mines, despite American protests. In 1915, Germany announced a U-boat blockade of the Allies’ ports and, in the ensuing months, killed a number of Americans in torpedo attacks on British vessels and one U.S. tanker. On May 7, 1915, a U-boat sank the British ocean liner Lusitania, killing close to 1,200 people, including 128 Americans. This event provoked an anti-German backlash in American public opinion, and, at Wilson’s encouragement, Congress passed the National Defense Act in 1916, which called for the buildup of military forces in anticipation of war—a policy known as “preparedness.”

After the Lusitania incident, Germany stopped attacking passenger ships for a few months. But in August 1915, Germany resumed attacks, sinking both British and French vessels. In 1916, when Wilson threatened to break diplomatic relations after one such attack, Germany responded with the Sussex Pledge, promising not to attack merchant ships without warning. This pledge eased the strain on U.S. neutrality for the remainder of 1916.

In 1916, Wilson was reelected on the slogan “He kept us out of war,” a tribute to his maintenance of American neutrality. Wilson and the Democrats portrayed the Republican Party as the party of war and uncertainty.