The United States declared war on the German Empire on April 6, 1917, nearly three years after World War I started. A ceasefire and Armistice was declared on November 11, 1918. Before entering the war, the U.S. had remained neutral, though it had been an important supplier to the United Kingdom, France, and the other powers of the Allies of World War I.

The U.S. made its major contributions in terms of supplies, raw material, and money, starting in 1917. American soldiers under General of the Armies John Pershing, Commander-in-Chief of the American Expeditionary Force (AEF), arrived at the rate of 10,000 men a day on the Western Front in the summer of 1918. During the war, the U.S. mobilized over 4 million military personnel and suffered the loss of over 116,000 soldiers.^[79] The war saw a dramatic expansion of the United States government in an effort to harness the war effort and a significant increase in the size of the U.S. Armed Forces.

After a relatively slow start in mobilizing the economy and labor force, by spring 1918, the nation was poised to play a role in the conflict. Under the leadership of President Woodrow Wilson, the war represented the climax of the Progressive Era as it sought to bring reform and democracy to the world. There was substantial public opposition to U.S. entry into the war.