Following WW2, US General George Marshall led a diplomatic mission to China to lead negotiations between the Kuomintang and the Communists to agree to peace and create a unified government. Before then, the two had been accusing each other of withholding aid supposed to be used to fight against the Japanese to prepare for when they would fight each other again. The negotiations gave time for both sides to continue to build up for the conflict and hurl more accusations at each other, which meant no progress was made, so the mission was a failure, Marshall went back to the States, and we know what happened next.
Later, General Douglas MacArthur called it "...one of the greatest blunders in American diplomatic history, for which the free world is now paying in blood and disaster...", and Marshall was targeted by Joseph McCarthy, who directly blamed him for the KMT's fall and the rise of a Communist Chinese state, alleging that "the Communists at that time were bottled up in two areas and were fighting a losing battle, but that because of those orders the situation was radically changed in favor of the Communists."
So that's the context out of the way. I'm no McCarthyist, and I do believe that the Marshall Mission was committed with the best of intentions, but let's suppose it never happened. Suppose that General Albert Coady Wedemeyer, at the time one of the more important American envoys in China and the commander of the US forces there, sent Washington a comprehensive report detailing that the Communists would fall, so Truman sees no need to send Marshall after concluding that the KMT would sweep the CCP rather than considering the situation too volatile at the gain of the Soviets alone, and that it could be ended easily. It doesn't have to be that POD, but the whole basis lies in Marshall never being sent to China and subsequently no truce being declared. The buildup to a resumed Chinese Civil War continues unabated, as the US continues sending the KMT aid. What happens?