Wouldn't the Chinese have been glad and became allies with the US after we defeated Japan? Instead less than ten years later the Chinese fought to support North Korea.
You're missing the biggest point here: the chinese civil war. China was divided since 1911 in several regions dominated by warlords, with a central republic dominated by the nationalistic Kuomintang Party and its leader Chiang kai shek. From 1927 another player came into the play, the chinese communist party, that openly fought the Kuomintang. Being routed from several cities and persecuted, in 1935 the communists started what become known as the "Long March", retiring all their forces and equipments in the north, towards the Manchurian/Japanese Frontier. Here, when the Japanese invaded China in 1937, they were one of the most reliable defences against the invasion, while the Kuomintang forces lost battle after battle and city after city, for several reasons. Here there was a real teuce between the two chinese factions, in order to fight the foreign invasor. And of course the USA helped the Kuomintang with aircraft and weapons, not the chinese communists.
That's why, when after the war the communists reprised and won the civil war, they had't really a good reason to thank the USA. Also, cold war and soviet influence.
That's why the USA refused to recognize the chinese communist government as the legitimate government of china for over 25 years.
That's why still today exist the Kuomintang funded Republic Of China, AKA Taiwan: after the defeat in mainland china, Kuomintang forces retired en masse on Taiwan Island, forming their own Chinese government and reclaming all of China as their rightful country.
The Chinese civil war is a fascinating and really complex argument, I highly suggest you to take a deep and through look at it, this was the really essential highlight.