Answer: Although many factors led to the defeat of the treaty, I would contend that it was primarily because of one of the articles of the charter of the League of Nations (President Wilson's proposal), which members of the Senate saw as a supranational and controlling force that would limit the power of the US government.
Explanation: Although President Wilson was a Democrat, since 1918 the Senate had started to be controlled by the Republicans. Wilson needed to be in good terms with Henry Lodge, the Senate Foreign Relations Committee Chairman, in order to receive support for the approval of the Treaty and his proposal of a League of Nations, but he was not. The animosity between Wilson and Lodge was in fact very notorious. In addition, Wilson did not invite any senators to the Paris Peace Conference in 1919. As a result, the Senate presented a series of reservations to the document, one of them being the rejection of one of the articles of the charter of the League of Nations, which compelled the members to preserve the territorial independence of the other members and to join their military forces, if necessary, in order to do so. Furthermore, Wilson's health started to debilitate, and he was not able to advocate for the Treaty any longer. Finally, several groups in the US, such as the German Americans and the Italian Americans, criticized the Treaty for various reasons - German Americans thought it was too harsh with their motherland.
