<P> During the Marco Polo Bridge Incident Japanese military demanded permission to enter the Chinese city of Wanping to search for a missing soldier . The Chinese refused . Later in the night, a unit of Japanese infantry attempted to breach Wanping's walled defences and were repulsed . An ultimatum by the Japanese was issued before they would declare war . The Chinese still refused . </P> <P> Although Private Shimura returned to his unit, by this point both sides were mobilising, with the Japanese deploying reinforcements and surrounding Wanping . The conflict then escalated further into a full - scale war . </P> <P> China fought Japan, with aid from the Soviet Union and the United States . After the Japanese attack on Pearl Harbor in 1941, the war merged with other conflicts of World War II as a major sector known as the China Burma India Theater . Some scholars consider the start of the full - scale Second Sino - Japanese War in 1937 to have been the beginning of World War II . The Second Sino - Japanese War was the largest Asian war in the 20th century . It accounted for the majority of civilian and military casualties in the Pacific War, with between 10 and 25 million Chinese civilians and over 4 million Chinese and Japanese military personnel dying from war - related violence, famine, and other causes . </P> <P> The war was the result of a decades - long Japanese imperialist policy to expand its influence politically and militarily in order to secure access to raw material reserves, food, and labor . The period after World War I brought about increasing stress on the Japanese polity . Leftists sought universal suffrage and greater rights for workers . Increasing textile production from Chinese mills was adversely affecting Japanese production . The Great Depression brought about a large slowdown in exports . All of this contributed to militant nationalism, culminating in the rise to power of a militarist fascist faction . This faction was led at its height by the Hideki Tojo cabinet of the Imperial Rule Assistance Association under edict from Emperor Hirohito . In 1931, the Mukden Incident helped spark the Japanese invasion of Manchuria . The Chinese were defeated and Japan created a new puppet state, Manchukuo; many historians cite 1931 as the beginning of the war . The view has been adopted by the PRC government . From 1931 to 1937, China and Japan continued to skirmish in small, localized engagements, so - called "incidents". </P>

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