The United States declared war on Germany on April 6, 1917, officially joining World War I. The U.S. was an independent power and did not officially join the Allies. It closely cooperated with the Allies militarily but acted alone in diplomacy.
The U.S. made its major contributions in terms of supplies, raw material, and money, starting in 1917. American soldiers under General John Pershing, Commander-in-Chief (C-in-C) of the American Expeditionary Force (AEF), arrived in large numbers on the Western Front in the summer of 1918. During the war, the U.S. mobilized over 4 million military personnel.
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, although there was substantial public opposition to U.S. entry into the war.