USA Enters World War I

In April 1917, the United States declared war on Germany and joined the Allied powers in World War I, ending nearly three years of official neutrality. The decision brought fresh manpower, money, and industrial capacity to a conflict that had exhausted Europe, and it marked a turning point in America's emergence as a global power.

When war broke out in 1914, President Woodrow Wilson called on Americans to remain neutral in thought as well as deed. The country had close economic ties to Britain and France, and American banks and manufacturers supplied the Allies with loans, munitions, and food. Germany, meanwhile, tried to break the British naval blockade with U-boats. The sinking of the British liner Lusitania in May 1915, which killed more than a thousand people, including over a hundred Americans, provoked outrage. Under U.S. pressure, Germany limited its submarine campaign, and Wilson won reelection in 1916 partly on the slogan that he had kept the country out of war.

That restraint collapsed in early 1917. Germany's leaders, gambling that they could starve Britain into submission before American power could matter, announced a return to unrestricted submarine warfare, in which ships could be sunk without warning. Wilson broke off diplomatic relations. Soon afterward, the British shared an intercepted message, the Zimmermann Telegram, sent by German foreign secretary Arthur Zimmermann. It proposed a German alliance with Mexico should the U.S. enter the war, hinting that Mexico might recover lost territory in the American Southwest. Its publication inflamed public opinion, especially since Zimmermann publicly acknowledged it was genuine. Further American merchant ships were sunk, and Russia's February Revolution removed the awkwardness of fighting alongside an autocracy.

On April 2, Wilson asked Congress for a declaration of war, famously framing the aim as making the world "safe for democracy." Congress approved within days, though not unanimously. The U.S. officially became an "associated" rather than allied power, preserving some independence of policy. The government introduced conscription, expanded the army and navy, and mobilized the economy. The American Expeditionary Forces under General John J. Pershing arrived in France in growing numbers, though they did not fight in large formations until 1918.

American entry helped tip the balance. Fresh troops bolstered the Allies as Germany launched its final offensives in spring 1918, and U.S. forces took part in the campaigns that led to the armistice that November. At home, the war brought expanded federal power, as well as restrictions on dissent, such as the Espionage and Sedition Acts. Wilson's postwar vision, including the League of Nations, was rejected by the U.S. Senate, but the war had permanently changed America's role in world affairs.