From 1904 on the japanese had a garrison army at corea, the Chōsen army.
Bit until the pacific war started in 1941, most japanese generals were against
the recruitment of corean nationals. Only a very few collaborants served for
japanese armed forces and reached a higher level of command at that time.
Their opinion changed in 1942, when it was decided to recruit corean people in big numbers.
The decision was mostly made because japan saw it would need more soldiers than their homeland could deliver.
So in 1942 it was decided to recruit coreans aswell, and started to sort out
young fitting coreans, to teach them japanese language and first combat
skills. In 1944 the recruitment finally started, and at least 130.000 coreans
were recruited in this year and joined the japanese forces. Still, the japanese were
very frightened that the coreans, now partially armed, could rebel against them.
Actually this plans for a rebel were made, young corean students and soldiers planned a rebel
against the japanese occupiers in 1944, but were arrested.
Even more coreans, and from an earlier point on - 1938, were forced to join japanese
industry. They worked especially in mines and weapon factories, but in food production aswell.
Just another small addition, japanese had soldiers from other asian areas aswell, such as
Taiwan, Hong Kong and Philipines.