jan 1, 1937 - Second Sino-Japanese War
In 1931 Japan took advantage of China's weakness in the Warlord Era and fabricated the Mukden Incident in 1931 to set up the puppet state of Manchukuo in Manchuria, with Puyi, who had been the last emperor of China, as its emperor. In 1937 the Marco Polo Bridge Incident triggered the Second Sino-Japanese War.
Added to timeline:
Causes of WWII