America as a whole became very isolationist after the end of the first world war, steering well clear of anything that would threaten peace. Wilson himself, as you might expect seeing as it was his brainchild, wanted to join the league but congress saw it as too risky to the future stability of the country. It would involve committing large parts of their army to solve disputes that may not even affect them and would also cost a lot of money. America would have been the main force behind the League but because they did not join, a lot of the time, the league was powerless to uphold its sanctions, such as banning oil to Italy during the Abyssinian crisis. Or actually getting things done, it took them over a year to rule Japanese invasion of manchuria as unlawful and they still didn't do anything about it.