Final answer:
The US declined to sign the Treaty of Versailles because the Senate did not want to join the League of Nations and felt that the provisions of the treaty were too harsh.
Step-by-step explanation:
The main reason for the US declining to sign the Treaty of Versailles was because the Senate did not want to join the League of Nations, which was established by the treaty. The Senate was concerned that joining the League of Nations would compromise their constitutional power to declare war and that the treaty would cost the United States their sovereignty. Additionally, some Americans felt that the provisions of the treaty were too harsh, and others believed that it was a sellout to imperialism.