Final answer:
The Xian Incident in December 1936 was the key event in which the Nationalist Chinese (GMD) and the Chinese communists (CCP) formed a united front to fight against Japanese aggression during World War II.
Step-by-step explanation:
The event that caused the Nationalist Chinese (GMD) and the Chinese communists (CCP) to unite to resist Japan was the Xian Incident. This event occurred in December 1936 when President Chiang Kai-shek was taken prisoner by Manchurian forces in Xian, China. This incident led to the creation of the Second United Front, with both the GMD and CCP joining together to oppose the Japanese aggression. Before the unification, the GMD and CCP were engaged in a civil war, but the threat posed by Japan's military actions, including the invasion of Manchuria and the subsequent incidents, shifted their focus to a mutual enemy. The Xian Incident stood out as a pivotal moment that paused the civil conflict and reoriented the two parties' efforts against a common foreign threat.