a. Joining the League of Nations
Even though Americans opposed many of the provisions of the Treaty of Versailles, they mainly opposed joining the League of Nations, and therefore it decided not to ratify the treaty.
For many Americans and the U.S Congress, entering the League would mean that they would have to participate in permanent economic-politic alliances that didn't serve their interests, thus breaking their isolationist policy that had only been briefly interrupted due to the war. The U.S. preferred to stay out of global affairs and focus on domestic issues instead.