Japan invaded China in 1937, creating what was essentially a three-way war between Japan, Mao Zedong's communists, and Chiang Kai-shek's nationalists. On 13 December that same year, the Nationalist capital of Nanking surrendered to Japanese troops. In the event known as the Nanking Massacre, Japanese troops massacred a large number of the defending garrison. It is estimated that as many as 300,000 people, including civilians, may have been killed, although the actual numbers are uncertain and the government of the People's Republic of China has never undertaken a full accounting of the massacres. In total, an estimated 20 million Chinese, mostly civilians, would be killed during World War II. A puppet state was also set up in China quickly afterwards, headed by Wang Jingwei. The second Sino-Japanese war would continue into World War II with the Communists and Nationalists in a temporary and uneasy nominal alliance against the Japanese.

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