This is a bit of an Ask Historians question.
I’ve been reading about the Japanese surrender on Wikipedia, and one thing I thought was strange was that the post-war occupation of Japan was largely handled only by the US under MacArthur. The original plan during the war was apparently for the Allies to divide it, but somehow the plan changed. Stalin allegedly wanted to occupy Hokkaido, Japan’s northern island, but Truman was opposed and it didn’t happen.
Contrast this to Germany (an East-West split than lasted for decades) and Austria (an East-West split, but the Soviets didn’t block full Austrian independence after a relatively short period. In Asia, the Japanese-controlled areas were mostly returned - China received Taiwan, coastal China in the south and east and Manchuria in the northeast. The Soviet Union retook Sakhalin island, just north of Hokkaido. Korea had been occupied by Japan for a few decades, and rather than Japan, it was Korea that was split between the Soviets and the US and shortly after became DPRK and ROK, transitioning into the Korean War as we know it, and the Korean peninsula is still split.
Japan, I think, fared reasonably well - the US were largely gone within ten years (but with a presence of military bases), and even during the occupation, Japan still technically governed themselves. I think it could have potentially gone much worse if the Soviets were involved, but the reasons for Soviet non-involvement are not very clear.
Pretty much this. When the news of the first atomic bomb reached high command, they treated it like any other bombing run. When news of the soviets formally declaring war on Japan reached them, there was an urgent meeting of the high council.
The USA did maintain a contingent of troops in Japan for some time, as did the UK, both to ensure Japan obeyed them and to make the soviets think twice before deciding to invade, as that would lead to a direct confrontation of USA x USSR. Japan was also used a staging point for USA troops some years later, when the Korean War began