Although World War I began in 1914, the United States did not join the war until 1917. 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 World War I broke out across Europe in 1914, President Woodrow Wilson proclaimed the United States would remain neutral, and many Americans supported this policy of nonintervention
United States did not concern itself with events and alliances in Europe and thus stayed out of the war. Wilson was firmly opposed to war, and believed that the key aim was to ensure peace, not only for the United States but across the world.