Why did Japan invade Manchuria in 1931?
The resources and the railroad network.
This is a fun story. So, to set the scene, Manchuria bordered the Japanese colony of Korea and had a lot of resources and an extensive railroad network. It was therefore vital that the Japanese control it, especially with their expansionist aims and increasing militarization leading into World War II. So, on September 18, 1931, Japanese officials blew up a section of their own railroad system, pretended the Chinese did it, called it Chinese aggression, and used it as a pretext to invade. They had taken control of the region by January 1932, and created Manchukuo, a puppet state nominally controlled by the last Qing ruler, Puyi.
(I call it a fun story because it's one of the pettier things to occur during/leading up to this war.)