Beginning in 1927, Mao Zedong and Zhu De grew the Communist Red Army from a small force of maybe 5,000 troops to over 250,000 troops by 1933. Despite the progressive antagonism and military attacks by the Nationalist forces, the communist learned to recruit for their party primarily from the peasants and the farmers. They even formed a State within a State called the Chinese Soviet Republic. It existed mostly in the southeast regions of China. There the Chinese communist party and the Red Army grew.
The Chinese Civil War started with the Autumn Harvest Uprising in 1927 and pitted the communist against the Nationalist. The war would last intermittently until almost 1950. After numerous attempts to destroy the communists by Chiang Kai-shek and his Nationalist forces, the Red Army and the communists escaped the southeast region of China. The Long March, as it is called, ended in 1935. It relocated the army and the communists several thousand miles to Shaanxi Province located in northwest China.
After the Xian Incident in late 1936, the Guomindang (Nationalists) and the communist agreed to a temporary truce to form the second united front. Its purpose was to the fight the ever increasingly aggressive Japanese encroachment into China.