There is absolutely ZERO reason for the aliens to return to earth. Revenge you say? They were defeated by a civilization incapable of traveling out of it's own solar system. And before you say, well look how the Taliban or Isis do us etc....That is nowhere near similar in scale and scope. An apt comparison would be more like, If the US decided to fight a war against the Dung beetle. I don't see the dung beetle having to much of a chance. BUT let's say the dung beetle somehow miraculously destroyed 15% of our army. Do you think we would continue to wage war against it? Secondly, what the fukk is up with this Alien civilization that they have such a hard on for our planet? They was bombing the shyt out of everything in sight, so they obviously don't want the planet. Ok so they are a war like civilization. Fine. But if you can travel the galaxy like they can, why fukk with Earth. Are they like a civilization of bullies. The fact that 2 different civilizations exist is more than likely proof that others exist as well. You telling me between whatever planet they come from which has to be further away then our observable universe and earth. They ain't found another planet or civilization to destroy? I mean it's got to cost a lot of money to travel a few light years away to come fukk with a planet for little to any reason.