I've answered this question before.

Aside from /u/ParallelPain I have to disagree with all the other answers in this thread. For the most part they seem to be treading the narrative of the Communist Party of China (CPC), which is to say that Chiang Kai-Shek's Nationalists were ineffective, corrupt, and did not fight well against the Japanese while Mao's Communists did.

If anything, it is generally accepted by historians, such as in Rana Mitter's Forgotten Ally, that the CPC was severely weakened by KMT offensives up to the Xi'an Incident, that the KMT bore the overwhelming brunt of the fighting, that Chiang was not in cahoots with the US to stomp out the Communists during the Second World War (if anything the US was considering assassinating Chiang for being a stubborn anti-Western dictator), and so forth. I talk a bit about the perverse historiography, as well as about the war itself, in detail here.