Final answer:
The Chinese Civil War between the Kuomintang and the Communists paused due to the Japanese invasion and the formation of the Second United Front, prompted by the Xi'an Incident in 1936 where Chiang Kai-shek was detained and urged to join forces against Japan.
Step-by-step explanation:
The Nationalists, known as the Kuomintang (KMT), led by Chiang Kai-shek, and the Communists, led by Mao Zedong, paused the Chinese Civil War primarily due to the Japanese invasion of China. This event led to what is known as the Second United Front, where both factions united against a common enemy. The invasion by Japan disrupted the ongoing conflict between the two Chinese parties, forcing them to realign their priorities to defend their nation. Despite this temporary truce during World War II, the underlying ideological and political tensions persisted, and the civil war resumed once the external threat from Japan was neutralized following Japan's defeat in 1945.
The Xi'an Incident in December 1936 is a notable cause for the ceasefire, where Chiang Kai-shek was detained by his own troops who sought an end to the internal conflict in light of the Japanese aggression. Chiang was compelled to form an alliance with the Communists to collectively oppose Japan, thus pausing the civil war.