When did the US formally enter WWI?

The United States declared war on Germany on April 6, 1917, over 2 1⁄2 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 Great Britain and the other Allied powers.

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 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 and suffered 110,000 deaths, including around 45,000 who died due to the 1918 Spanish Flu outbreak (30,000 before they even reached France). 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, although there was substantial public opposition to U.S. entry into the war.