how did they get into power?
because the japanese invasion causes great casualties for the nationalist?
the CCP has great military strategy? the aid from soviet?
what did the role of usa play?

supermal-ga:

The rise of communism in China took place over the period from 1920
through the 1950's. There were a lot of contributing factors, enough
to write a book about. In fact, if you check out some university
libraries (depending on where in the world you are located), you might
be able to find:

Schwartz, Benjamin I. "Chinese Communism and the Rise of Mao". Harvard
University Press

In a nutshell, a combination of socio-economic conditions up to,
during, and after WWII, perceived and actual corruption and cronyism
in the Nationalist gov't, and a lot of idealism resulted in the
peasant revolution that drove the Communists to power, and drove the
Nationalists out of the country.