World War I, Great Depression, and World War II

At the outbreak of World War I in 1914, the United States remained neutral. American sympathies were with the British and French, although many citizens, mostly Irish and German, were opposed to intervention. In 1917, however, they joined the Allies, helping to turn the tide against the Central Powers. Reluctant to be involved in European affairs, the Senate did not ratify the Treaty of Versailles, which established the League of Nations. Instead, the country continued to pursue a policy of unilateralism, verging at times on isolationism. After seven decades, the women's rights movement won passage of a constitutional amendment in 1920 granting women's suffrage. In part due to the service of many in the war, Native Americans won U.S. citizenship in the Indian Citizenship Act of 1924.

During most of the 1920s, the United States enjoyed a period of unbalanced prosperity as farm profits fell while industrial profits grew. A rise in debt and an inflated stock market culminated in the 1929 crash that, combined with the Dust Bowl, triggered the Great Depression. After his election as president in 1932, Franklin Delano Roosevelt responded with the New Deal, a range of policies increasing government intervention in the economy. The nation would not fully recover from the economic depression until the industrial mobilization spurred by its entrance into World War II.

On December 7, 1941, the United States was driven to join the Allies against the Axis Powers after a surprise attack on Pearl Harbor by Japan. World War II had a greater economic cost than any in American history, but it helped to pull the economy out of depression by providing much-needed jobs and bringing many women into the labor market. Allied conferences at Bretton Woods and Yalta outlined a new system of intergovernmental organizations that placed the United States and Soviet Union at the center of world affairs. As victory was achieved in Europe, a 1945 international conference held in San Francisco produced the United Nations Charter, which became active shortly after the war's end. The United States, having developed the first nuclear weapons, used them on the Japanese cities of Hiroshima and Nagasaki in August. Japan surrendered on September 2, ending the war.