From ROI Wiki

Crassia is the homeworld of the Crassians, descendants of a Roman village, abducted from Earth by the Ichigwa, experimented on, and then dumped on Crassia to hide what they'd done from the rest of the universe. But, despite this, the Romans of Earth who were dumped here survived, grew, and became strong, but never lost their desire to conquer all of Earth, and eventually the entire universe.