The Smithsonian just put a few more documentaries on Netflix. There is one in particular that is about the training of the first US spies, trained by Britain, in Canada.
In short, one segment speaks on how an American spy named Frankie Gleason, in the OSS, trained at Camp X. It says he helped Chinese Nationalists retake large swathes of their homeland back from an enormous push by Japan.
If this is true, how come it's not commonly known like other things Allies did in WW2?
Why is our [US] relationship so rough with China these days?
As far as your second question (why is the US relationship with China so rough if they helped the Chinese during the war) goes, the short answer is that the US backed the wrong horse, as it were, when it came to the Chinese civil war.
The US (and most of the Allies, save the USSR) supported the Nationalist/Kuomintang forces. Unfortunately for the US, the Kuomintang were forced out of China by the Communist forces in 1949, and reestablished the government of the Republic of China on Taiwan. The People's Republic of China doesn't view the Taiwanese government as legitimate, but they're working toward a reunification of the two countries under the One-China Policy. Of course, the RoC doesn't view the PRC to be the legitimate government of China, and is pursuing their own One-China Policy with themselves as the ruling force in China.
So the TL;DR is that the USA backed the wrong horse in a civil war and the horse they didn't back remembers it.