China: Japanese troops using a heavy machine gun during the invasion of Manchuria in 1931.

The Second Sino-Japanese War is usually dated from 1937 to Japan's final defeat in 1945, but in fact Japan and China had been in a state of undeclared war from the time of the Mukden Incident in 1931 when Japan seized Manchuria and set up the puppet state of Manchukuo. The Japanese installed the former Qing Emperor Puyi as Head of State in 1932, and two years later he was declared Emperor of Manchukuo with the era name of Kangde ('Tranquility and Virtue'). Manchukuo would remain in Japanese hands until the Red Army swept across the frontier from the Soviet Union in 1945, at the end of the Second World War, and routed the Japanese in several weeks of bitter fighting.