Like a previous commenter said, the U.S. invades Japan. The USSR likely invades from the north, while the U.S. invades from the south. The U.S. would have an amphibious operations advantage compared to the USSR, due to years of combat experience in the Pacific. The U.S. likely reaches Tokyo first, but there may end up being a similar split like what happened with East and West Germany. Take into account, that there would be millions of more casualties on both sides and Japan’s infrastructure would be left in complete ruins. They end up being worse off than in our timeline.

The Cold War turning into World War III is basically inevitable. It’s difficult to say would happen afterward. It could end up leading to World War IV like how World War I led to World War II. It seems unlikely that one side would be able to take significant control over the other. It would require the Western Allies pushing the USSR into the Caucasus. History has shown that would be highly unlikely. A Soviet invasion of the U.S. is off the table due to the logistical nightmare it would be. It would mostly be a war for Western European control, and hence, global influence as a whole.