Why did the United States not join the League of Nations?

The United States did not join the League of Nations primarily due to opposition in the U.S. Senate, which refused to ratify the Treaty of Versailles containing the League's covenant.123

Key factors in this rejection included:

• Concerns Over Sovereignty: Many senators, led by Henry Cabot Lodge, feared that Article 10 of the Covenant—which mandated collective security—would force the United States into foreign conflicts without congressional approval, thereby violating the U.S. Constitution.143
• Political Obstruction: Lodge and other isolationists introduced reservations to the treaty to limit U.S. commitments. President Woodrow Wilson, however, insisted on an all-or-nothing approach and refused to accept these compromises.143
• Presidential Health: Following a debilitating stroke in 1919, Wilson’s ability to negotiate with the Senate was severely impaired, leading to a rigid stance that contributed to the treaty's final defeat in March 1920.23

Ultimately, the Senate failed to reach the two-thirds majority required for ratification, effectively ending the possibility of U.S. membership.243