America only looks out for america only when they are threatened do they stand to be counted why?

they let millions die with no regard for europe in world war two claiming they were preventing a world war then pearl harbour occured and they had no problem destroying parts of japan with nucleur war killing innocent people and destroying the land and people suffer innocent children and women where were the rules of war there they would not help when hitler was geting started in his main power seat in the late 1930,s and early 1940,s why does america a superpower only help when it suits them despite the fact that the americans are mostly europeans migrated britain made many mistakes then but at least they took the chance to attack