War was launched against China after the Marco Polo Bridge Incident of July 7, 1937, in which an allegedly unplanned clash took place near Beiping (as Beijing was then called) between Chinese and Japanese troops and quickly escalated into full-scale warfare. The Second Sino-Japanese War (1937-45) ensued, and relations with the United States, Britain, and the Soviet Union deteriorated.

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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...

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.