Final answer:
The U.S. Senate opposed joining the League of Nations to avoid foreign entanglements, preserve U.S. sovereignty, and prevent compulsory military commitments. Concerns over Article X and potential European wars, coupled with President Wilson's lack of consultation with senators, led to the refusal to ratify the treaty.
Step-by-step explanation:
The opposition by the U.S. Senate to join the League of Nations was primarily due to the desire to avoid foreign entanglements and preserve U.S. sovereignty. Key concerns included the fear that American troops could be committed to conflicts around the world without Congressional approval, thus undermining the constitutional power to declare war. Senator Henry Cabot Lodge and other Republicans were particularly wary of Article X of the League Covenant, which called for collective security but was seen as a potential source of endless military commitments.
In addition, the Treaty of Versailles and the proposed League of Nations were perceived by many as dangerous, with the potential to drag the United States into another war in Europe. The Republican-dominated Senate at the time was isolationist in foreign policy views and desired to avoid any obligations that could arise from such a collective security agreement. President Woodrow Wilson's failure to consult with key senators during the peace process compounded the issue, leading to a lack of support for the treaty's ratification.