regirock, japan is vastly stronger than korea in the late 1800s
japan occupied you, these rebel forces are likely not an organized military but rather an angry militia driven by "we might win independence" alone.
japan, a by far superior nation, is occupying you already. these disorganized rebel forces are likely nothing more than an annoyance that makes the occupying force's job harder. japan already holds the fortifications; they have the superior ground when on defense and on offense. also numbers, training, equipment, everything.