Corollary: If you do end up taking cattle (or sheep or pigs or whatever), you had better make sure that you either do it quietly or have indestructible ships, as farmers tend to be rather protective of their bovine (and ovine and porcine) property, and rural folk are the most likely in any country to be highly armed (particularly in the places that raise the most livestock).And if you are looking for a bite to eat, you might want to try abducting large, meaty animals instead of wasting your time with the boniest, least nutritious species on the planet -to say nothing of the only species on the planet capable of understanding what you're doing and taking exception to it. Harvesting native intelligent beings as cattle is dreadfully inefficient, and they tend to object. If you're sufficiently advanced to cross interstellar space in the first place, consider investing some research into basic technologies like food replication.In other words, if the most dangerous species on your target planet is still very much in the creature stage, or at best, tribal stage of intelligence, you've pretty much already won the war before it even began. Given those odds, chances are pretty good that there's another life-bearing planet a similar distance away that you could easily colonise without conquering. If it's a life-bearing planet you're after, keep in mind that ours didn't evolve a sapient species until the most recent 0.004% of its existence.In fact, if you don't need it for reaction mass and do need it to drink, also see #1 (sure, it's dirty, but you can clean it up with simple and comparatively cheap distillation-and probably get some nice salts, hydrocarbons, etc., in the bargain). If you need it for reaction mass and not to drink, see #1. Even a fleet of a thousand mile-wide spaceships can't hold enough to make a significant impact on a planetary biosphere.
Aluminum? Uranium? Go mine an asteroid and leave the Puny Earthlings alone. note Not "multiple Earths" as in "many times the amount of water on Earth" "multiple Earths" as in "many times the Earth's entire mass". Water is the commonest compound in the universe even if you happen to live on Tatooine, your solar system probably contains multiple Earths' worth of water in its comets alone. If you're looking for some ridiculously common substance like helium, water, oxygen, etc., there are millions of lifeless rocks where you can get the stuff essentially for free, without the need to fight off pesky hostile natives.If you're after a material good, is there an easier way to get it? Now is also the time to do some basic research on the target world in question, to make sure it's even worth the effort of conquering. To begin with, put some thought into why you're invading in the first place.