The US set up their own government in Japan. It was a democracy and with the spread communism throughout the world the US wanted an ally in Asia that they could basically use as their own military base to stop the spread of communism. Also the enemies of the US in Japan were basically gone. The majority of the soldiers that posed a threat killed themselves after the Emperor told them to surrender. As for the Soviet Union they completely saw what the US did in Japan and this angered them and they continued to try and spread communism. However the relations between the 2 countries were never good. Both just knew that they needed the other in order to win the war and because after the war they no longer needed each other, they just split and went their separate ways. Think of it as a bully and the kid he bullies in a class together. They don’t like each other at all but they might just work together because they don’t like the teacher or don’t want to do the assignment or whatever. Basically the US and Soviet Union just had a common enemy in Germany and that’s all that ever brought them together.
Hope that helped!