PC: American Intervention in the Chinese Civil War?

Is it plausible for the US Armed Forces to become involved in the Chinese Civil War, obviously on the side of the KMT, in a significant way?
At what phase of the Chinese Civil War are you referring to? IIRC, around 50,000 Marines were used to secure strategic sites for the KMT following WW2 in OTL. Although they weren't involved in any actual fighting.
I have lately been considering my own TL about foreign involvement in the early phases of the Chinese Civil War. My PoD was going to be the Nanking Incident of 1927.

Wikipedia said:
The Nanjing incident, or Nanking incident occurred in March 1927 during the capture of the city by Communist forces from the Nationalists.

A Communist army was marching south to take the city which was a center of foreign trading activity as well as the capital of Nationalist China. When the Communists neared the city, the overwhelmed Nationalist officers deserted so riots broke out as the enlisted men began to retreat but about 10,000 were trapped within the city without transportation to leave while over 70,000 Communists prepared to enter.

Nationalists who were supposed to defend the city began burning houses and attacked the British, American and Japanese consulates, killing the British and Japanese consuls in the process. Uniformed Communists executed the American vice president of Nanking University, Doctor John Elias Williams. Soldiers of the 6th Army systematically looted the homes and businesses of the foreign residents. One Japanese naval officer was killed along with at least one Frenchmen and one Italian.

The Chinese forces stormed the consulates of America, Britain, and Japan, looting nearly every foreign property and almost assassinating the Japanese consul. An American, two British, one French, an Italian, and a Japanese were killed by Chinese Nationalist forces. Chinese snipers targeted the American consul and marines who were guarding him, Chinese bullets forced them to flee into Sacony Hall where American citizens were hiding out, one Chinese soldier declared, "We don't want money, anyway, we want to kill."

In fact, in reaction to this incident (and to the deliberate targeting of foreign nationals in the city, including diplomatic staff), France, Britain, and other powers wrote up an ultimatum to send to the KMT. However, Japanese Foreign Minister Shidehara was quite pro-KMT at the time (he kept pushing for Chinese tariff autonomy, against the will of the other powers) and he refused to support it. It fell through.

If Shidehara was not FM, and/or if the incident is worse (more diplomatic staff are murdered), then I think it would be easy to get a foreign punitive expedition into China. Things could just spiral out of control from there.
IMHO it's unlikely. Segments of US politics were somewhat sympathetic to the Chinese cause, but none of those people were influential enough to involve the USA in an actual war, certainly not over a Sino-Japanese War. In WWII the GMD is too far gone to effectively fight or desire to fight the Japanese, while the USA's unlikely to be in any good strategic position to give them aid of the sort that would be most helpful to them, on the Asian mainland.
There were a number of incidents in the late '40s where US Marines stationed in northern China were killed or captured by Chinese Communist forces, but they were overlooked by Truman as a necessary expense if the US was to avoid escalating the conflict to the point of Soviet involvement. The Chinese Communists were also pretty good about negotiating for the release of US prisoners in those few incidents, so it wasn't really enough to attract the ire of the American leadership.

However, you could easily blow the whole affair up by having one of those incidents result in the sinking of a US Navy vessel with great loss of life. At the time, the US Navy had a substantial amount of the Pacific Fleet committed to aiding the KMT with ferrying their troops from southern to northern ports in addition to helping the KMT train their naval and air forces. Perhaps it may be plausible for the communists to sabotage one of the transport vessels or perhaps a destroyer escort by means of a USS Cole style attack. From there, you might accrue a significant number American deaths, resulting in a public outcry of some sort. But importantly, the inciting incident will have to be some irregular/asymmetric warfare attack, since the US Navy and marines would just preemptively withdraw from a northern Chinese port whenever it was clear the communists would be able to seize it soon.