So Japan was an Imperialist nation, and the US was annoyed with their behaviour in China.
Japan wanted to create a 'Greater Asian Co-prosperity Sphere' - ie an Empire in Asia where the European didn't have colonies.
The US couldn't stand for this. Their ambitions were to be a great power, and to spread the whole way across the pacific. But they also couldn't declare war with Japan over the invasion and human rights absues in China.
Europeans were busy, so there was an opportunity to expand into east asia. And the result of the war was a defeated Japan leaving a power vacumn in Asia. It would be filled by the US and by various Communists.
Infact, from what i'm reading, most of the later wars in East Asia/South East Asia, were a result of the former major colonial powers (Japan, France, and the British and Dutch) being unable to hold onto their empires.
The US swept in, occupied Japan, allied with Some Koreans, invaded Vietnam (on the pretext of preventing the spread of Communism, when the Vietnamese probably hated China more than anyone else, and certainly weren't going to kow-tow to the PRC... Infact, China invaded Vietnam after the Americans left, and were likewise defeated)
I never really saw this little bit of history in such terms.