Your invaders will probably have an ulterior motive for zapping your green and pleasant land back to the Stone Age. Usually, it is because they love the environment, like the alien hippies from The Day The Earth Stood Still. This is bad, because humanity has already ruined its eco-structure, but also good because it's an easy subject to feign an interest in. Pull a sad face, hug a tree and promise to change your ways - they'll get the message. Suckers.