In 1931 Japan took advantage of China 's very weak central government in the Warlord Era and fabricated the Mukden Incident in 1931 to set up the puppet state of Manchukuo in Manchuria . Puyi , who had been the last emperor of China , became " emperor " of China again ; he was a Japanese puppet . In 1937 the Marco Polo Bridge Incident triggered the Second Sino-Japanese War . The invasion was launched by the bombing of many cities such as Shanghai , Nanjing and Guangzhou . The latest , which began on 22 and 23 September 1937 , called forth widespread protests culminating in a resolution by the Far Eastern Advisory Committee of the League of Nations . The Imperial Japanese Army captured the Chinese capital city of Nanjing , and committed war crimes in the Nanjing massacre . The war tied down large numbers of Chinese soldiers , so Japan set up three different Chinese puppet states to enlist some Chinese support
war crimes
Japanese war crimes
Diplomatic history of World War II