The “Mukden incident” of 1931 – commemorated in China every year as an act of Japanese aggression – saw Japanese troops blow up a railway in northeast China as an excuse to take over Manchuria.
Japan had already been expanding its territory in Asia militarily for several decades.
After defeating China in 1894-95 it established itself in the Korean peninsula, Taiwan and southern Manchuria, and by winning a 1904-05 war with Russia it solidified control of Korea, which it later formally colonised.
Tokyo had a particular interest in resource-rich Manchuria, a strategic location and an industrial and transport hub – benefits that seemed increasingly vital given global economic troubles at the time.
Japanese officials in Manchuria also felt growing pressure to assert control of the area to thwart rising popular dissatisfaction with their presence and increasing closeness with China “proper” south of the Great Wall.
The ethnic Han Chinese population had grown in recent decades through immigration to the region, home to the indigenous Manchus.
Meanwhile Chinese Nationalist leader Chiang Kai-shek had unified China and in 1928 Manchurian warlord Zhang Xueliang pledged loyalty to his government.
Japanese military officers in Mukden – today the city of Shenyang in Liaoning province – took matters into their own hands, despite hesitation from the government in Tokyo.
They blew up a portion of the Southern Manchurian railway, blamed the incident on Chinese citizens and used it as a pretext to assert control across the region.
By 1932 they had taken over Manchuria and turned it into the puppet state of Manchukuo, nominally headed by the Chinese emperor Pu Yi but controlled by Japanese officials.
The region remained isolated from the rest of China until Japan was defeated in the second world war.