Based on

Movies like Hero Commandos (1979)

In the early stages of WW2 Japan invaded China. Obviously historical accuracy was not paramount in the movie creators minds. The Japanese set up a puppet government in Manchuria to act as a centre from which to conquer the rest of China. As part of their strategy the Japanese headquarters in Manchuria plans to process a large quantity of illegal opium in a short period. With that amount of drugs floating around temptation was close by. The money raised from the opium processing plant is to be used to fund the military expansion. But not all goes to plan.