What could have Korea done to resist japan?

The Korean military was in an inactive state at the time of the Japanese invasion.

Explanation:

The government of Joseon organized its army in a system of five guards that means five armies were posted at various corners of the kingdom, and it was compulsory for all qualified men of the lower and middle class to serve in the military for a few years. From a military perspective, Korea's situation was completely opposite that of Japan.  Since Joseon was in extended peacetime, military preparedness had deteriorated significantly. By 1592 it had become common to bribe officials to escape out of service in the military.