And second, for much of the time that Guomindang was trying to crush the CCP, significant portions of China were being occupied and or invaded by Japan. Thirdly, the communists were just better at fighting the Japanese than the nationalists were, in spite of the fact that Chiang Kai-shek had extensive support from the US. And each time the nationalists failed against the Japanese, their prestige among their fellow Chinese diminished. And it wasn't helped by nationalist corruption or the collection of onerous taxes from peasants or stories about nationalist troops putting on civilian clothing and abandoning the city of Nanking during its awful destruction by the Japanese army in 1937. Meanwhile, the communists were winning over the peasants in their northwestern enclave by making sure that troops didn't pillage local land and giving peasants a greater say in local government. Now, that isn't to say that everything was rosy under Mao's communist leadership, even at its earliest stages. By the way, that is an actual chalk illustration.