League of Nations
The League of Nations was an international organization, existing from 1919 to 1946. The League was created by the the Allies at the Paris Peace Conference in 1919 to provide a mechanism to prevent future wars. During World War II, the League was replaced in 1945 by the UN.
The United States never joined the League of Nations. The primary reason for the absence of the United States in the League of Nations was that the American public thought that it was another way of getting entangled in European affairs. This idea was best represented when President Woodrow Wilson, a strong advocate for the League of Nations, attempted to ratify the League of Nations Charter via the United States Senate which resulted ultimately in a failing vote.