The US under the early months of the Truman administration was certainly concerned with communism, but perhaps not to the same dramatic extent as what would come later in Korea and Vietnam. Something important to note is the idea of worldviews over time--US policymakers and the State Department was simply not as wedded to the idea that communism needed to be stopped everywhere regardless of strategic value.

The first thing to note is that the US was involved in the Chinese Civil War, from military advisors to supplying weapons to the KMT. Much of the US support infrastructure had its roots in US-KMT cooperation during WWII. However, there were early doubts about the effectiveness of the KMT and the virulent corruption in the Nationalist government. The CCP, on the other hand, was a plucky group of fighters which some US journalists (most notably Edgar Snow) lauded for their fighting ability against the Japanese. This is important to note: the KMT may not have been Communist, but their circle of admirers in the US was relatively small.

US military officials wanted to broker a truce between the CCP and KMT after WWII; an effort which failed by 1946. After that point, the KMT was always on the back foot, with the CCP --with its superior morale, support base, military efficacy, and bolstered by captured Japanese weapons and equipment--quickly taking cities and sending the KMT into a declining spiral in its ability to maintain a front line.

The US still provided some support in the form of military aid, transporting and training KMT troops, and even sending some troops to strategic areas in Shandong and Hebei Province to protect American property and facilitate the repatriation of Japanese nationals stuck in China. Most China experts on staff in the State Dept were highly (and rightfully) skeptical that more aid could turn the tide, given the level of corruption in the KMT government, poor military, and utter absence of popular support. After the election of the 1948, the Truman administration had essentially lost faith in Chiang's government completely and saw it as a lost cause, refusing to pour more money into a bottomless hole. Chiang and his govt subsequently flee to Taiwan. Essentially, Truman's foreign policy calculus was fairly straightforward: this was a Chinese Civil War where the US-supported side was clearly losing by a landslide, so there was not a strategic or rational reason to try and take a one in a million chance of averting the ultimate outcome.

But this final result of the war, with a Communist Chinese government in power, was politically devastating for Truman. "Who Lost China?" became a direct attack by US conservatives against the administration, in a moment that was increasingly governed by American anxiety about confrontation with the Soviet Union (the Soviets get their own A-bomb in 1949, which is very important in understanding why the Communist threat suddenly seemed so much more omnipotent for many Americans). China experts in the State Department and academia, who had rightfully pointed out the deficiencies in the KMT government and predicted eventual Communist victory, came under attack from conservatives, with some chased out from their positions. The false charges from Sen McCarthy that Owen Lattimore, a leading China expert in US academia, was a Soviet spy became a central example of how fear about Communism had begun to inflame anti-Communist sentiment in the US and clamp down on the range of views that were politically viable at the time.

The Chinese Civil War indirectly would promote future US interventions in other conflicts in Asia, including Korea, because the notion of a president being responsible for the "fall" of a non-communist state was set in place by the blame leveled at Truman for the defeat of the KMT.

A good (and relatively simply written and enjoyable) account of US involvement in the Chinese Civil War is Kurtz-Phelan's The China Mission, which follows General Marshall's attempts to broker a peace and also charts much of the intricacies of US-KMT-CCP relations that I have briefly summarized here.