After establishing the puppet state of Manchukuo, the Japanese began to push from south of the Great Wall into northern China and into the coastal provinces. Chinese fury against Japan was predictable, but anger was also directed against the Kuomintang government, which at the time was more preoccupied with anti-Communist extermination campaigns than with resisting the Japanese invaders. The importance of "internal unity before external danger" was forcefully brought home in December 1936, when Nationalist troops, having been ousted from Manchuria by the Japanese mutinied at Xi'an. The mutineers forcibly detained Chiang Kai-shek for several days until he agreed to cease hostilities against the Communist forces in northwest China and to assign Communist units combat duties in designated anti-Japanese front areas.

On January 6, 1941, Kuomintang forces breached the Communist- Kuomintang anti-Japanese truce, attacking and destroying the headquarters of the Communist New Fourth Army, with thousands of Communist casualties; the truce held, but it was clear that the civil war would quickly resume when the Japanese threat subsided.

In August 1945, following the Japanese surrender, Communist Chinese forces occupied much of formerly Japanese-occupied north China. In September 1945, 53,000 American marines occupied Peking and many strategic north Chinese centers, as the civil war resumed and Kuomintang-Communist fighting became general.

In January 1946, General George C. Marshall's attempt to mediate the Chinese Civil War resulted in a temporary truce, but it did not hold.