The previous posts were quite strong in their analysis. In some respects, the premise of the question presumes that American forces backing anti- Communist forces were of the same magnitude and force as the Communist ones. If this premise is accepted, then, indeed, the Americans did "lose" China to the Communists. However, there is a line of logic that presumes that the Americans simply believed that China would defeat the Communist, and underestimated the pull of Mao and his Communist Revolution. In underestimating it, the Americans might not have "lost" China, but simply underestimated the winning side and pull of the Communists Either way, it is fair to say that emerging from the victory of the Second World War, American democracy was seen to have carried over around the world and many probably felt it would have been naturally extended into China in 1949. As proven, this was not the case.
Agreed. Another argument was that some say we failed to recognize the true threat of the communist rebellion. The Nationalists under Chiang Kai Shek had scored some major victories against the rebellion (See: The Long March) so America's government may have thought, at the time, that China could hold its own with the aid we had already given them.
Given how aggressively we would soon move in the Korean War, some thought we should have done so in defense of the non-communist government of China a bit earlier than that. Our foreign policy of containment developed by George F. Kennan was still taking shape in 1949, however, so I don't know if I would lay all the blame at the feet of Truman, who was rightfully preoccupied with Europe, the new Soviet nuclear threat and the a new NATO/Warsaw Pact standoff.
I am not completely sure which arguments you mean, so I will give you a few things that were said back then. Your teacher probably expects to see something specific from your book or lectures, though.
The major argument they made was that China was ours to lose. They argued that China was naturally our ally and that it should never have become communist.
A second major argument they made is that we lost China because the US government was too soft on Communism. They argued that Democrats like those in Harry Truman's administration were the next best things to Communists themselves. Because of this, they said, the government did not care if the Communists took China. That, they argued, was why we "lost" China.