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.
I had a friend once who felt that aliens invading earth to steal our water would be a great idea for a story. "The universe is made almost entirely of hydrogen, most of what's left over is oxygen. Water is the easiest thing in the universe to get." Yeah, but the other planets don't have any. "Yeah, but you don't *need* to go to a planet to get water, you can just make it with a Busard ramjet in space." Well, yeah, but it's easier to get it off earth. "No it isn't. It took six MILLION pounds of rocket and fuel to put 36,186 pounds on the moon. That's the equivalent weight of 4338 gallons of water, or 1338 pounds of rocket to the gallon. In terms of money, in 1969 bucks...about $168,750 to the ounce. That's about $1,041,872 in today's money.
Hard to imagine a drought that would justify that high a price.
"Well, they could use space man technology."
Eh, fine whatever. Some years later another friend told me the reason he felt we hadn't colonized space was that our governments feared that colonies on planets around other stars would hurl rocks at earth ACROSS STELLAR DISTANCES, and strike earth, causing damage. I explained how there were any number of things wrong with this: Stars are really far away, even at light speed, hitting a moving target a few quadrillion miles away with a rock is unlikely, and if you've got the energy to MOVE a rock across interstellar space in the first place, then you've got the energy to simply blow up any planet you want, and not muck about with such things.
He told me "I didn't get it."