As someone who was in Tientsin as a Vice Consul at the time it was taken by the Communists, Gordon Tullock can explain the collapse of Nationalist China:
The Nationalists led by Chiang Kai Shek and with much Russian aid drove north from Canton and took the lower Yangtse Valley. It should be emphasized that not only was the Nationalist army given much aid by Russians, but also it was then allied with the Chinese Communist Party. Further, much of the territory nominally under control from Nanking was actually ruled by local warlords, some of whom rejoiced in formal commissions as local governors.
At this point, Stalin revealed his normal paranoia. He ordered the Communists in China to overthrow the then national government and take formal power. This was very badly timed and planned with the result that the Chinese Communists were literally beheaded on the mud flats outside Shanghai. There were a few high-ranking Communists who were safe in Moscow, and some lesser Communists holding small areas in south central China. Among these later was Mao Tse Tung.
The Communists in south central China made their way to the poor, lightly populated area around Yenan, near the Russian border:
The Communists stayed in Yenan, and the Japanese, who talked about anti-Communism, made no real effort to eliminate them. Presumably they knew that the sensible thing to do when your enemies quarrel is to help the weaker side.
The Japanese left many large pockets of China, between the railroads and the rivers, to the Chinese:
These areas remained under Chinese control and the Communists seized some of them. Incidentally, the western press referred to the Communists as guerillas and either did not mention the other Chinese forces in the other unoccupied areas or called them bandits.
The Communists and the “bandits” mainly left each other alone. Both types of Chinese, after December 7, thought that the United States would win the war for them and hence did little fighting with the Japanese.
As Japan collapsed, the Communists moved into Manchuria, where the Russians supplied them with Japanese arms:
The Nationalists then moved north and invaded Manchuria. The Communists tried to stop them and at Su Ping Kai the Nationalists won a major victory. The United States quickly slammed an arms embargo on the Nationalists. What led General Marshall to do this has never been explained. Its ostensible objective was to force the Nationalists to form a coalition government with the Communists. At this time, preventing such coalitions in France and Italy was a major objective of American foreign policy.
Since they opposed the “corrupt” regime, the Communists were obviously all right.