The U.S. Senate refused to ratify the Treaty of Versailles after World War 1 for several reasons. One of the main concerns was the inclusion of the League of Nations, which was seen by some senators as a potential threat to U.S. sovereignty. Others were worried about the financial and military obligations imposed by the treaty. Additionally, President Woodrow Wilson's political opponents opposed the treaty and saw it as an opportunity to challenge his authority. These factors, combined with other political and ideological disagreements within the Senate, led to the treaty's rejection.