Nothing because it wasn't his fault and who knows if the world may even be worse if it wasn't for hitler. What would have happened when the Cold War started regardless, and there hadn't been a major war America had been heavily involved in fighting. People wouldn't know how the bloodshed felt, and may be more likely to dive into it.
And before you say "but America helped during WW1", financially YES but in terms of their military they saw little action and were regarded by the other troops as useless.