The League of Nations was proposed by Woodrow Wilson in Article XIV of his Fourteen Points, forming the basis of the Covenant of the League of Nations that was part of the Treaty of Versailles. However, the treaty was never ratified by the U.S. Senate. Among the concerns of senators was Article X of the treaty, which required all members to preserve the territorial independence of all other members. Violations could be met with joint action among nations against aggressors. A strong group of politicians and citizens objected to the U.S. military participating with those of other nations, preferring that the United States maintain independence and free decisionmaking in employing military forces.

Without treaty ratification, the United States never became an official member of the League, which existed from 1920 to 1946. American diplomats assisted in League activities and attended meetings, but...