The US was quite involved in China during the 30's and 40's. Predominantly, their objective there was to roll back Japan. Which worked, just like how the Soviets bled out the Germans on the Eastern Front, the Chinese bled out the Japanese on their homeland. They were perennially pissed at Chiang and his government for fighting the Communists instead of the Japanese and general incompetence. They funded the Nationalist Army with a lot of equipment, but obviously not troops since, well, the Chinese could never really run out of troops haha. Indirectly they funded the Communists as well, since there were quite a few mass retreats where US-aided weapons would be captured, as well as because of guerrilla tactics that necessitated that enemy equipment be captured, since the Communists at that time had very little industrial capability. It's the same thing we see happening in Syria - you arm your side, and when they surrender to ISIS, they take all the weapons you've donated. IIRC the US was interested in peace in China, and so pressured the Nationalists to form a coalition government with the Communists after the Japanese were defeated. I'm surprised that the US wasn't actually even more interventionist - like how they divided Germany and Korea - but then again I don't think they were looking forwards to get bogged down in China, which in 1945 was still a fifth of the world's population.
so tl;dr not really involved in the Civil War per se, mostly there to help China fight the Japanese, obviously supported the Nationalists but otherwise stayed out of the internal politics as much as they could though.
I just researched after and found that the US stationed over 150,000 troops in China. It sounds like a lot, but keep in mind that this is just post WWII and I'm sure a lot of those troops were transferred from the Pacific front.