ANSWER: It made the US obligated to defend other nations.
The League of Nations, established on January 10, 1920, was an international organization adopted by over 63 countries in which the United States decided to not be part of it. The organization's main goal was to maintain world peace by settling disputes through negotiation and arbitration and to have support from all members in case of a war waged on one of the countries.
Isolationists objected to the League of Nations because it would obligate Americans to defend other nations. The U.S. wanted to stay out of global affairs and avoid any type of alliances that didn't serve American interests. They decided to focus on domestic issues instead.