In the First World War, Japan joined the Allied powers, but played only a minor role in fighting German colonial forces in East Asia. At the following Paris Peace Conference of 1919, Japan's proposal of amending a "racial equality clause" to the covenant of the League of Nations was rejected by the United States, Britain and Australia. Arrogance and racial discrimination towards the Japanese had plagued Japanese-Western relations since the forced opening of the country in the 1800s, and were again a major factor for the deterioration of relations in the decades preceeding World War 2. In 1924, for example, the US Congress passed the Exclusion Act that prohibited further immigration from Japan.
During the 1930s, the military established almost complete control over the government. Many political enemies were assassinated, and communists persecuted. Indoctrination and censorship in education and media were further intensified. Navy and army officers soon occupied most of the important offices, including the one of the prime minister.
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After WW1, Japan's economical situation worsened. The Great Kanto Earthquake of 1923 and the world wide depression of 1929 intensified the crisis.
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Already earlier, Japan followed the example of Western nations and forced China into unequal economical and political treaties. Furthermore, Japan's influence over Manchuria had been steadily growing since the end of the Russo-Japanese war of 1904-05. When the Chinese Nationalists began to seriously challenge Japan's position in Manchuria in 1931, the Kwantung Army (Japanese armed forces in Manchuria) occupied Manchuria. In the following year, "Manchukuo" was declared an independent state, controlled by the Kwantung Army through a puppet government. In the same year, the Japanese air force bombarded Shanghai in order to protect Japanese residents from anti Japanese movements.
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In July 1937, the second Sino-Japanese War broke out. A small incident was soon made into a full scale war by the Kwantung army which acted rather independently from a more moderate government. The Japanese forces succeeded in occupying almost the whole coast of China and committed severe war atrocities on the Chinese population, especially during the fall of the capital Nanking. However, the Chinese government never surrendered completely, and the war continued on a lower scale until 1945.
On December 13, 1937, Japanese soldiers entered Nanking, then the capital of China. During the assault, Japanese soldiers killed people at random; raped women, girls and boys; bayoneted people tied to stakes; used Chinese peasants as human minesweepers; and looted and set fire to shops, temples, houses and churches.
Japanese soldiers raped thousands of girls and women, many of whom were dragged from their homes. By the end of December, 20,000 cases of rape had been reported. One girl was raped 37 times. Another had her four-month-old son smothered by the soldier who raped her. Some Japanese soldiers raped pregnant women, killed them, cut the fetuses out of their bodies and then had their picture taken with the fetuses. Some young Chinese women disguised themselves as elderly women to escape being raped.
One former Japanese soldier, who confessed to sexually assaulting a Chinese woman with a wooden sword, said "I kept beating her until her skin broke and started to bleed, but she didn't answer my questions." A soldiers that ate the flesh of a young Chinese boy said, "It was the only time, and not so much meat."
Photographs taken by Japanese show Imperial army soldiers holding up severed heads; placing, their feet on dead women and babies; rape victims begging for mercy; and soldiers standing beside dead people hung from ropes as if they were prize fish. Some Japanese soldiers competed among themselves to see who could kill the most Chinese. Two sub-lieutenants, battling to be the first to reach 100, beheaded 167 people in a single day.
The slaughter lasted for six weeks. One relief agency buried 100,000 people; the Red Crescent buried 43,000. In just five days, the Japanese disposed of 150,000 bodies by throwing them in the Yangtze.
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The turning point in the Pacific War was the battle of Midway in June 1942. From then on, the Allied forces slowly won back the territories occupied by Japan. In 1944, intensive air raids started over Japan. In spring 1945, US forces invaded Okinawa in one of the war's bloodiest battles.
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On August 14, however, Emperor Showa finally decided to surrender unconditionally.