Heh heh, this ties in to my college major, so it's time for my book learning to pay off!
Chiang Kai Shek never had a chance after WWII. At that time, the CCP (Chinese Communist Party) had a larger, more experienced army with better equipment and overwhelming popular support. Before WWII, he might have been able to pull it off if he only had 1 or 2 major problems, but the had 3: Economic collapse, the CCP, and the encroaching Japanese.
During WWII itself, Chiang lost the military advantage and popular support because of his strategy of avoiding direct confrontation with the Japanese. Instead, his armies would always retreat and pull back, mostly grouping around major inland cities. This makes sense because his armies would have been anhilated by the superior Japanese forces, plus the Japanese never had intentions of fully taking over mainland China anyways. Unfortunately, it looks really bad to the public when your armies are always retreating and losing. This also wasn't so good for his troops' morale, which led to a lot of them running away. On the other hand, Mao Zedong mounted guerrilla warfare against the Japanese. He never really made a dent militarily, but was seen as a hero by the Chinese people and could recruit people in droves because he fought on against impossible odds when the government would turn and run. Also, CCP forces captured a lot of abandoned GMD equipment and used it in their own military, giving them the military advantage after WWII.
What one of my professors argued is that we should have actually supported the CCP against the GMD. A group of State Department and US Army officials sent to Yunnan to spend time with Mao and his forces stated in their report that the CCP would eventually win over the GMD, and that it would be better to back them. But of course that report was ignored.
Believe it or not, the Soviet Union was actually backing the GMD against the CCP, because the CCP and USSR had a bad falling out over their differing Marxist ideals. Arguably, the US should have seen the USSR, not Communism in general, as the enemy and backed the CCP to give us an ally against the USSR.