First off this was a terrible decision on the U.S parts.
Explanation:
But, the reason why Congress refused to sign off on the bill to become a member of the League was due to isolationism. Which was the foreign policy of staying neutral. Most americans felt like they didn't want the U.S actively participating in foreign countries. But as we all know this was a disaster for the League since they were to rely heavily on the U.S as a leader. During WWII it was obvious the League was too timid to stop German military advancements and held no power over world peace.
