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 parties, 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 outlined 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 consulted; 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.

Return to Civil War

The wartime policy of the United States was initially to help China become a strong ally and a stabilizing force in postwar East Asia. As the conflict between the Nationalists and the Communists intensified, however, the United States sought unsuccessfully to reconcile the rival forces for a more effective anti-Japanese war effort. Toward the end of the war, United States Marines were used to hold Beiping and Tianjin against a possible Soviet incursion, and logistic support was given to Nationalist forces in north and northeast China.

Through the mediatory influence of the United States a military truce was arranged in January 1946, but battles between Nationalists and Communists soon resumed. Realizing that American efforts short of large-scale armed intervention could not stop the war, the United States withdrew the American mission, headed by General George C. Marshall, in early 1947. The civil war, in which the United States aided the Nationalists with massive economic loans but no military support, became more widespread. Battles raged not only for territories but also for the allegiance of cross sections of the population.

Belatedly, the Nationalist government sought to enlist popular support through internal reforms. The effort was in vain, however, because of the rampant corruption in government and the accompanying political and economic chaos. By late 1948 the Nationalist position was bleak. The demoralized and undisciplined Nationalist troops proved no match for the People's Liberation Army (PLA or ). The Communists were well established in the north and northeast. Although the Nationalists had an advantage in numbers of men and weapons, controlled a much larger territory and population than their adversaries, and enjoyed considerable international support, they were exhausted by the long war with Japan and the attendant internal responsibilities. In January 1949 Beiping was taken by the Communists without a fight, and its name changed back to Beijing. Between April and November, major cities passed from Guomindang to Communist control with minimal resistance. In most cases the surrounding countryside and small towns had come under Communist influence long before the cities. After Chiang Kai-shek and a few hundred thousand Nationalist troops fled from the mainland to the island of Taiwan, there remained only isolated pockets of resistance. In December 1949 Chiang proclaimed Taipei (), Taiwan (), the temporary capital of China.