The entire basis for the invasion is that the Aliens have a strict code against killing. Here’s how it works:
(A Scene at the Alien Homeworld)
Leader: You! Yes, you! Who are you?
Random Alien: I’m a Second Undersecretary for Colonization.
Leader: Yes, you’ll do perfectly. Go to the Recruitment Center. We need volunteers.
RA: Volunteers?
Leader: For our Glorious Invasion of Earth!
RA: Will it be dangerous?
Leader: Dangerous? Of course! The aboriginals have primitive but deadly weapons, and the planet is covered with poisonous dihydrogen oxide. But if it weren’t dangerous, there would be no glory! Now get going!
(On board the Flying Saucer, hovering over a field)
Pilot:OK, you “volunteers”, get out.
RA: But nobody told us what to do!
Pilot: You’re supposed to make Crop Circles.
RA: With what?
Pilot: Take those Boards, stakes, and Rope.
RA: I thought we used the backwash from our Star Drive.
Pilot: What? Are you nuts? Get out there and make circular patterns.
RA: OK, but why?
Pilot: They’re …uhhh… signal markers. Yeah, that’s it. They point the way to the major cities.
RA: But I can see the cities from here. They light up the whole sky!
Pilot: Are you questioning the High Command? Get out there and make Circles!
(On Board the Big Ships, just before the invasion)
High Commander: I want you Aliens to go out there and attack those houses. Bring back specimens for the glory of our planet!
RA: But we haven’t had any military training!
HC: You won’t need any. You can outrun and outjump any of those earthlings. And you’ve got those poison gas implants.
RA: But they onmly have a range of six inches! Their guns can shoot farther than that!
Second Random Alien: Don’t we get any suits?
Third Random Alien: Don’t we even get any clothes!?
HC: What are you? Aliens? or Wimps? You oughtta be able to lick those Earthings with one hand tied behind your back! You don’t need suits against Dihydrogen Oxide. It would just …uhhh… it would just trap that deadly stuff against your skin. Now get out there and fight!
(On the Saucers, returning to the Alien Homeworld)
High Commander: Well, how did the Harvesting go?
Pilot: We sent out 10,500 troops.
HC: How many returned to the ships?
Pilot: 134
HC: Sounds like a pretty good harvest to me. Maybe now there’ll be a little elbow room.
Pilot: At least until the next Harvest.