Alien invasions make for great stories, but they don't make one lick of common sense. People hear about D-Day, and they think, "Wow, spacemen could do that! Or we could do that to a spaceman planet! In space!" But invading another world isn't like hopping a plane and jumping out, there's enormous energies involved, carefully calculated math, long travel times. If you've got the energy to travel across the stars, frankly, you don't *need* anything Earth could offer you, not even as raw resources.