The Chinese had fought a war with Japan in the late 19th Century, which had left them resentful and humiliated.
Following peace settlements at the end of WWI, Shandong province in China was taken from Germany and handed to Japan, with the Chinese left angry that they had not regained land that they saw as rightfully theirs. This again influenced some revolutionary groups. The disillusion that many felt at the Treaty of Versailles actually led to increases in membership of the CCP.
Russia, who had already gone through a revolution provided help for Chinese revolutionaries, and their Communist revolution made the Chinese more aware of the Marxist ideas that would influence two of the largest revolutionary parties (GMD and CCP).
The Japanese invasion from 1937 onwards provided respite for the CCP forces, who were able to retrain and regain numbers.
The Japanese invasion also created a United Front that prevented for some time, the GMD attacking the CCP.
The Japanese invasion attracted foreign intervention. The Russians and the USA intervened. The Russians provided support for the Communists, while the USA wanted establish peace after they dropped atomic bombs in Japan to stop their expansion in China.
With Japan defeated, the USA provided aid to the Nationalists after attempts at forming a coalition failed. The Nationalists however failed when they were outmanouvered by the CCP forced who had support of ex-GMD members and peasants. Their Guerrilla tactics proved extremely important, and despite GMD’s support from the USA, the communists emerged victorious.
It could be argued then that the USA did not significantly influence the outcome, otherwise the GMD would have won the civil war, however without the