The US aided the British because we are allies, that is why countries had alliances, it was the precursor to the UN, and Japan got involved to eliminate the US fleet of Carriers, they were unfortnate in attacking a place where the carriers weren't, so the US mobilized all the carriers, and setup for a massive invasion, but realizing the defensive positin Japan had, and knowing what it needed for Normandie, the US opted to save US soldiers, an resources and sent Enola Gay with Fat man and Little Boy to avoid excessive loss of US resources.