The United States rejected the Treaty of Versailles due to the opposition of a group of senators called the Irreconcilables, who believed that under the terms of the treaty, the United States would lose too much of its autonomy to the League of Nations. All of the Irreconcilables were enemies of President Woodrow Wilson, who originally advocated for the League of Nations and helped compose the details of the treaty.
In 1919, the Treaty of Versailles reached the Senate for a vote of ratification. Most Democrats supported the treaty, but the Republicans were divided. Besides the Irreconcilables, a second group of Republicans called the Reservationists, led by Senator Henry Cabot Lodge, declared they would support the treaty if certain alterations were made. When Lodge formed a coalition with pro-treaty Democrats and submitted a revised treaty with 14 amendments to the Senate, Wilson persuaded the Democrats to reject it. The final Senate vote fell far short of the two-thirds majority needed to ratify the treaty. It was the first time the Senate ever rejected a peace treaty.
Because the U.S. Senate never ratified the Treaty of Versailles, the United States signed separate treaties with Germany in 1921 and 1922 that enabled the United States to help Germany rebuild as a nation apart from the strict supervision of the League of Nations.