18 September 1931. A small explosion on a Japanese-owned railway outside Mukden, in Manchuria. Officers of the Kwantung Army, Japan's garrison along the line, set the charge themselves and blame Chinese troops. By morning they are taking the city. Japan's war in China starts with a staged blast.
Why then
Japan has held the railway zone since beating Russia in 1905. Officers in the field have a theory of Japan's destiny and a government in Tokyo that cannot quite sack them. China is split between Chiang's Nationalists, regional warlords and Communists, and the world economy has just collapsed.
What they wanted
The Kwantung Army is after a puppet state and a resource hinterland. Chiang chooses to fight the Communists first and Japan later. Tokyo's civilians would like the army reeled in without looking weak. The army does not reel.
What changed
Manchuria becomes Manchukuo, with China's last emperor, Puyi, as its figurehead. The League sends a commission; when its report blames Japan, Japan walks out in 1933. Six years later the full-scale war begins.
Still argued
Louise Young shows the empire in Manchuria was sold to the Japanese public as a mass cause, not only hatched by rogue colonels. How far Tokyo's own policy drove them is still argued.