Hungry for raw materials and pressed by a growing population, Japan initiated the seizure of Manchuria in September 1931 and established ex-Qing emperor Puyi as head of the puppet regime of Manchukuo in 1932. The loss of Manchuria, and its vast potential for industrial development and war industries, was a blow to the Nationalist economy. The League of Nations, established at the end of World War I, was unable to act in the face of the Japanese defiance. 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 Guomindang 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 (who had 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.

The Chinese resistance stiffened after July 7, 1937, when a clash, occurred between Chinese and Japanese troops outside Beijing (then renamed Beiping) near the Marco Polo Bridge. This skirmish not only marked the beginning of open, though undeclared, war between China and Japan but also hastened the formal announcement of the second Guomindang-CCP united front against Japan. The collaboration took place with salutary effects for the beleaguered CCP. The distrust between the two par-ties, however, was scarcely veiled. The uneasy alliance began to break down after late 1938, despite Japan's steady territorial gains in northern China, the coastal regions, and the rich Chang Jiang Valley in central China. After 1940, conflicts between the Nationalists and Communists became more frequent in the areas not under Japanese control. The Communists expanded their influence wherever opportunities presented themselves through mass organizations, administrative reforms, and the land- and tax-reform measures favoring the peasants---while the Nationalists attempted to neutralize the spread of Communist influence.

At Yan'an and elsewhere in the "liberated areas," Mao was able to adapt Marxism-Leninism to Chinese conditions. He taught party cadres to lead the masses by living and working with them, eating their food, and thinking their thoughts. The Red Army fostered an image of conducting guerrilla warfare in defense of the people. Communist troops adapted to changing wartime conditions and became a seasoned fighting force. Mao also began preparing for the establishment of a new China. In 1940 he out-lined the program of the Chinese Communists for an eventual seizure of power. His teachings became the central tenets of the CCP doctrine that came to be formalized as Mao Zedong Thought. With skillful organizational and propaganda work, the Communists Increased party membership from 100 000 in 1937 to 1.2 million by 1945.

In 1945 China emerged from the war nominally a great military power but actually a nation economically prostrate and on the verge of all-out civil war. The economy deteriorated, sapped by the military demands of foreign war and internal strife, by spiraling inflation, and by Nationalist profiteering, speculation, and hoarding. Starvation came in the wake of the war, and millions were rendered homeless by floods and the unsettled conditions in many parts of the country. The situation was further complicated by an Allied agreement at the Yalta Conference in February 1945 that brought Soviet troops into Manchuria to hasten the termination of war against Japan. Although the Chinese had not been present at Yalta, they had been consul-ted ; they had agreed to have the Soviets enter the war in the belief that the Soviet Union would deal only with the Nationalist government. After the war, the Soviet Union, as part of the Yalta agreement's allowing a Soviet sphere of influence in Manchuria, dismantled and removed more than half the industrial equipment left there by the Japanese. The Soviet presence in northeast China enabled the Communists to move in long enough to arm themselves with the equipment surrendered by the withdrawing Japanese army. The problems of rehabilitating the formerly Japanese-occupied areas and of reconstructing the nation from the ravages of a protracted war were staggering, to say the least.