The idea of aliens conquering earth for sole purpose of collecting resources seems pretty ridiculous to me. Given the scale of planetary evolution, they are much more likely to be a million years more advanced than us than a couple hundred. There would be so many celestial bodies out there rich in resources for such a civilization to choose from, why wouldn't they just take one of the millions of bodies that doesn't have nuclear weapons on it?
The point wouldn't be to collect resources. It would be to ensure that those monkeys with nukes and star drives don't try to do something bad. They won't try to steal our water. They'll just try to wipe us out, probably with bombardment from distances far beyond orbit.
