You're a spacefaring civilisation capable of building an interstellar invasion fleet, and you're living on planets? Planets - those huge objects with totally predictable orbital paths, at the bottom of a deep gravity well? The ones that you might as well paint a big red target on and shout 'POINT THE RELATIVISTIC KILL VEHICLES HERE, GUYS!' And where 99.99999% of the usable mineral resources are miles down and under fantastically high pressure? Those things?
Wow. You know, I'd have thought that the art of creating artificial habitats in space would be one that a civilisation would master long before they get to the point of interstellar colonisation and conquest. I guess I was wrong.