Final answer:
The unification of the GMD and the CCP to fight Japan during World War II, particularly following the Xian Incident, acted as the catalyst for the CCP's increased involvement in the fight for China.
Step-by-step explanation:
The catalyst for the Chinese Communist Party (CCP) to join the fight for China was the Japanese aggression during World War II. The existing civil war between the CCP and the Guomindang (GMD) or Nationalist Party was interrupted by a larger threat: the Japanese invasion. Specifically, the Xian Incident in 1936 forced the GMD's leader Chiang Kai-shek to forge the Second United Front with the CCP, uniting both parties against the common enemy of Japan. After the war, the Cold War politics influenced further international relations, with the USSR supporting Mao and the United States assisting the GMD.