>>512172606 (OP)
Letting Russia invade and occupying half of Japan and making it communist for 40 years.. leading to World War III with the Soviet Union and no nukes right after.
The nukes ended the war before the Soviets could take a lot of japanese clay, and paused the idea of WW3 because of atomic bomb uncertainty long enough that we never had a hot direct war.
No other thing could have prevented WW3 with the USSR... than them not knowing how many bombs we had that could do that devastation to them... and us not knowing if they too had the bomb and could hit us back.
That uncertainty held things together until we knew for sure they had the bomb and then the arms race created deterrence.