The conflict between the Kuomintang and the Communist party started in August 1927 - December 1936, and then March 1946 - May 1950. ( they teamed together to fight off the Japanese who were invading from 1936 - 1945
The leader of the Kuomintang was Chiang Kai-shek, while the leader of the Communists was Mao Zedong. Before the civil war, the communists and nationalists were part of the same government, and worked together to defeat the chinese warlords that still ruled certain parts of China. However, after winning against the warlords, the Nationalists turned against the Communist and started to systematically destroy them. A group of Communists led by Mao Zedong retreated into the rural mountain while the Nationalists chased after them. The Nationalists never quite caught up, and the communists were able to gather strength there. Communism became more and more popular as the corrupted officers of the Kuomintang mistreated many peasants, which made them all become ethuistastic about the Communist government, and they started to support them. After a string of defeats, the Nationalists retreated into Formosa, or present-day Taiwan. There the US helped them set up a Nationalist government. The Communist government then controlled all of mainland china, and started to brainwash people into thinking that Communism and Russia was Great, and that America and Democracy & Nationalism was horrible. Then Mao Ze Dong did a number of mistakes that costed people lives. #1, he forced farmers off their lands and made them live in communions, so that they could interact and help other people [and also try to increase food production] This failed however, because the farmers weren't motivated inside to farm the land. (#1, it wasn't there land, and because of that #2, they weren't able to keep anything they grew and were not able to sell the crops to make money). #2, in his great leap forward, he tried to increase farm production and military, however, he forgot about other things, which left China lagging behind in many aspects, including manufacturing of non-military equipment, and most of every-day items. [ Remember, because they were Communists, their trading partner was pretty much Russia. Russia was happy to trade with them in pretty much everything EXCEPT Military secret, weapons, and technology.] This would then lead to Chinese trying to gain independence from the communist regime and democracy later on.
[sorry had to shorten it, theres a limit to the amount of words you can type in it]
hope this helps