why do you think americans supported participation in the UN after world war 2 when they had opposed participation in the league of nations after world war 1?

It had become clear that isolationism could not put the country out of war as whatever happened abroad always came to haunt them locally. Examples such as the war of 1812, and the world war 1 were a perfect example of how isolationsim as apolicy could not gurantee world peace and hence, the UN as an intsrument that would enforce this peace was imperative.