Thumpalumpacus
Major
Operation Unthinkable should begin immediately, and the plans for war with Japan would have to be radically changed - up to a separate peace with the transfer of Northern Sakhalin and the Kuril Islands to Japan on condition of participation in the war against the USSR.
Geopolitically, this would be an interesting alternative - at the same time, the Japanese would be forced to fight against the Chinese Communists.
But from the point of view of justice and humanism, of course, it is an extremely ugly alternative.
There's no way on Earth the American people would accept any sort of negotiated peace with the nation that bombed Pearl Harbor, aside from retaining the Emperor, of course.