Insect Empire Yoma
The Yoma are a race of insectoid humans that lived on Terra II long before humans colonized it. They dominated the planet and surrounding solar system up until 2000 years ago. A Cosmic storm of unknown origin wiped out the Yoma outposts on the other planets and forced the leaders underground. They buried two mechs that would awaken the Yoma after they were activated. This would signal that the Cosmic storms had passed, and they could restart their empire.
Sure enough, the Fire Legion did find the two mechs and activated them. The Yoma are furious that so much time has passed and that "lesser beings" have colonized their planet. They hope to exterminate them and make their empire greater than it once was.