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Conflict between China and Japan

In 1931, without declaring war, Japan invaded Manchuria, seeking raw materials to fuel its growing industrial economy. By 1937, Japan controlled Manchuria and it was ready to move deeper into China. The Marco Polo Bridge Incident on 7 July 1937 provoked full-scale war between China and Japan. The Nationalist Party and the Chinese Communists suspended their civil war in order to form a nominal alliance against Japan, and the Soviet Union quickly lent support by providing large amounts of materiel to Chinese troops. In August 1937, Generalissimo Chiang Kai-shek deployed his best armies to fight about 300,000 Japanese troops in Shanghai, but, after three months of fighting, Shanghai fell. The Japanese continued to push the Chinese forces back, capturing the capital Nanjing in mid-December 1937 and committing atrocities in the Nanjing Massacre, including rape, murder and torture.

In March 1938, Nationalist forces won their first victory at Taierzhuang, but then the city of Xuzhou was taken by the Japanese in May. In June 1938, Japan deployed about 350,000 troops to invade Wuhan and captured it in October after a four-month campaign. The Japanese achieved major military victories, but world opinion—in particular in the US—condemned Japan, especially after the Panay incident. In addition, the Japanese had failed to destroy the Chinese army, who continued to resist from their new capital in Chongqing, or in the case of the Communists, Yan'an.

In 1939, Japanese forces tried to push into the Soviet Far East. They were defeated in the Battle of Khalkhin Gol by a mixed Soviet and Mongolian force led by Georgy Zhukov. This stopped Japanese expansion to the north, and Soviet aid to China ended as a result of the signing of the Soviet–Japanese Neutrality Pact. In September 1940, Japan decided to invade French Indochina, which was controlled at the time by Vichy France. On 27 September Japan signed a military alliance with Germany and Italy, becoming one of the three main Axis Powers.

The war entered a new phase with the defeat of the Japanese at the Battle of Suixian–Zaoyang, 1st Battle of Changsha, Battle of Kunlun Pass and Battle of Zaoyi. After these victories, Chinese nationalist forces launched a large-scale counter-offensive in early 1940; however, due to its low military-industrial capacity, it was repulsed in late March 1940. In August 1940, Chinese communists launched an offensive in Central China; in retaliation, Japan instituted the " Three Alls Policy " ("Kill all, Burn all, Loot all") in occupied areas, killing at least 2.7 million civilians.

By 1941 the conflict had become a stalemate. Although Japan had occupied much of northern, central, and coastal China, the Nationalist Government had retreated to the interior with a provisional capital set up at Chongqing while the Chinese communists remained in control of base areas in Shaanxi. The Japanese found its aggression against the retreating and regrouping Chinese army was stalled by the mountainous terrain in southwestern China while the Communists organised widespread guerrilla and saboteur activities in northern and eastern China behind the Japanese front line.

Japan sponsored several puppet governments, one of which was headed by Wang Jingwei. Conflicts between Chinese Communist and Nationalist forces vying for territory control behind enemy lines culminated in a major armed clash in January 1941, effectively ending their co-operation.

Japanese strategic bombing efforts mostly targeted large Chinese cities such as Shanghai, Wuhan, and Chongqing, with around 5,000 raids from February 1938 to August 1943 in the later case. Japan's strategic bombing campaigns killed 260,000–350,934 non-combatants.