Answer:
The American entry into World War I came in April 1917, after more than two and a half years of efforts by President Woodrow Wilson to keep the United States out of the war.
The U.S. entered World War I because Germany embarked on a deadly gamble. Germany sank many American merchant ships around the British Isles which prompted the American entry into the war.
The impact of the United States joining the war was significant. The additional firepower, resources, and soldiers of the U.S. helped to tip the balance of the war in favor of the Allies. When war broke out in 1914, the United States had a policy of neutrality.