What made Chinese citizens during the 1940's side with either the CCP (Chinese Communist Party) or the KMT (Guomindang, or Nationalists)?
The Chinese Communist Party was extremely popular in the 1930s and 40s with the peasantry, the poorest Chinese citizens who had known hardship and privation for generations. Wisely, the CCP made it a point of providing food and basic supplies to the poor, knowing that, with their numbers and basic support, the insurgency could become more successful. One specific method they used was promising peasants they could take land from the former landlords if the revolution was successful. By the late 1940s, they oculd mobilize hundreds of thousands of supporters.
The Kuomintang was aggressively and extensively backed by the United States, just as the CCP was backed by the Soviet Union and China became an early Cold War battleground. More than $4 billion in aid from the United States flowed to the Nationalist government, some of which was used to provide relief and employment among the population. The Kuomintang also exploited regional and ethnic rivalries and divisions in order to recruit support.