The American entry into World War I not only made the war truly global in nature but also interjected the nation directly into the political affairs of Europe. Initially neutral, but with loyalties divided along immigrant backgrounds, the United States joined the Allies. German submarine warfare violated American neutrality and foreshadowed the economic losses which would accompany an Axis triumph. Congress declared war on April 6, 1917.

Once war was declared, Congress reacted by providing troops through the Selective Service Act of 1917 and focusing on internal loyalty with the Espionage Act of 1917 and Sedition Act of 1918. The economic boom caused by the war ended in the bust of 1919, but was followed by the increasingly heady economic boom of the 1920s.