The United States never joined the League of Nations as a result of the decision by the US Congress not to ratify the Versailles Treaty. This refusal to join the league also fitted in with America's desire to have an isolationist policy throughout the world.
The U.S. before World War 1, and immediately after did not want to be involved in European problems. Many still felt we could be isolated from the problems of the world, so they did not want to join a union of foreign governments. answers.yahoo.com