There have been many times when the Earth has been at the mercy of alien invaders. And there have been more than a few times where haven't — not because of our weapons, our savagery, or our inner strength — but because the aliens themselves were morons. Here are 14 alien invaders who did not think their invasions through.
1) It, It Conquered the World
The titular It actually had a reasonable solid plan for taking over the planet: Send a legion of Venusian bats to bite people, causing them to fall under It's mental control, while disrupting the world's electricity so all those that haven't been bitten yet can't mount an offense. However, It failed to mind-control Lee van Cleef, who played the scientist who helped bring It to Earth, because Lee was initially on his side (one of those humans need to get rid of their emotions before we kill each other kind of things). This inattention to detail comes back to bite It in the ass when Lee changes his mind about It, which leads to It's other major problem: Being a huge, squat cucumber alien hiding in a cave with no protection whatsoever. If you're vulnerable to a single man wielding a flamethrower, maybe conquering the world is a bit beyond you.
2) The Psyclos, Battlefield Earth
Whether you're talking about the book or the movie, the Psyclos of L. Ron Hubbard's Battlefield Earth do manage to conquer Earth, and hold it for 1,000 years. Not bad. But it all goes to hell because in both cases, Terl the Psyclo Security Chief basically handed humanity a loaded gun. See, the air the Psyclos breath basically explodes when it comes in contact with radioactive material. In the movie, Terl fast-educates a human named Johnny Goodboy Tyler, gives him a ship, and sends him out unsupervised to go find gold; Johnny instead finds a shit-ton of unused military vehicles and weapons, teaches people to use them, and leads an attack against the Psychos, even teleporting a nuclear bomb to Planet Psyclo, destroying it completely. In the book, Terl is even stupider; he forces a bunch of humans to mine gold that he can't reach because the mine also has uranium in it. Terl basically says, "Here is a mine full of gold and the Psyclo's kryptonite; please bring me only the gold." The humans, in a movie that could have surprised no one but Terl, mine the uranium instead, Terl teleports it to Planet Psyclo, and Planet Psyclo completely blows up. Invasion over.
3) The Saucer-Men, Invasion of the Saucer Men
While the title of the movie says they're invading, it's very unclear how the Saucer-Men were planning on conquering the planet. Especially given that the first interaction between the Saucer-Men and humanity is when a couple of teens hits one of the aliens with their car and then drives off. The body is picked up by Frank Gorshin (he played the Riddler in Batman '66) who sticks it in his fridge, as anyone would do when discovering the corpse of an alien, and eventually a few more Saucer-Men show up at Frank's house and stick him with their needle-fingers, which basically shoot a lethal amount of booze into him (there are worse ways to go). Eventually, a bunch of teens track down the aliens, shine their cars' high-beams at them, and the Saucer-Men die. Look, any alien invasion that can be thwarted by a bunch of teens from the '50s is an incompetent alien invasion.
4) The Slitheen, Doctor Who
All the terrible alien invaders featured in Doctor Who would take up its own list, so we'll limit ourselves to the Slitheen, the reviled aliens who have secretly invaded Earth for a variety of get-rich-quick schemes, all of which involve destroying the planet. The Doctor stopped their first plan with a single missile, then stopped the second by turning the sole remaining Slitheen to an egg. The third was stopped by Sarah Jane Smith's kid Luke, who convinced the Slitheen to reset their machinery, thus destroying it. Note: The Slitheen are most known for their constant farting while in disguise as humans, which hasn't contributed directly to their pathetic and easily achievable downfalls, but is terrible anyways.
5) The Martians, The Sirens of Titan
When the Martians — actually brainwashed, mind-controlled humans — invade Earth, they are outnumbered, outgunned, and outplanned from the very beginning. They are so easily slaughtered that humanity actually starts to feel bad for them, which is exactly what Martian war instigator Winston Niles Rumfoord planned. He's then able to start a new religion the "Church of God the Utterly Indifferent", which he thinks is going to unite humanity to some glorious future, but is really just part of a larger by a group of aliens from Tralfamadore to get humanity to make a small part a stranded alien needs for his spaceship.
6) The Aliens of Plan 9 from Outer Space
Jesus, where to even begin? Okay, the aliens of Ed Wood's most infamous movie want to destroy humanity because they're probably going to discover solarbonite, a substance that can "explode sunlight molecules." In order to destroy humanity, then, the aliens hover over various TV station headquarters, try to start a conversation with the U.S. military (although god knows why, since they plan on killing everybody anyways), and apparently destroy a small town somewhere? And that's before the aliens implement Plan 9, which causes the resurrection of the dead — not a bad idea, if you were able to resurrect more than a shapely vampire girl, Tor Johnson, and the world's worst Bela Lugosi stand-in. Eventually, the aliens invite a bunch of humans into their ship, the humans start a fight, something catches on fire, and the aliens try to fly their flaming ship away but it explodes. Just poor invasion management all the way around, really.
7) The Skrulls, Marvel Comics
The shape-shifting Skrulls have gotten better at invading Earth — sure, their Secret Invasion failed, but it was a long-term plan reasonably well-executed and required pretty much the combined forces of Earth's superhero to stop — but their first invasion of Earth didn't go nearly so well. In the second issue of Fantastic Four, four Skrulls came to Earth to prepare the planet for invasion; job #1 was pretending to be the Fantastic Four and being assholes, to ruin the FF's reputation and… this was somehow going to prevent the FF from stopping the entire Skrull Armada. The FF merely flew to the Skrulls' commander, pretending to the Skrull agents, and told them that conquering Earth seemed like it was going to be hard. The Skrulls left immediately. To add insult to injury, the FF also hypnotized the four original agents into thinking they were (shape-shifting into) cows.
8) The Krankorians, Prince of Space
A group of aliens who resemble chickens and very obviously don't wear underwear arrive at Earth in their spaceship, and declare the Earth conquered. They are defeated by a single man, without superpowers, because the Krankorians weapons are for some reason useless against him (thus implying their weapons are useless against all humans). Part of the Krankorians downfall is that their leader, the Phantom of Krankor, has one plan, and that's continuing to fire his useless weapons against the Prince of Space in hopes that they suddenly, for some unknown reason, start working. In the original Japanese movie, titled Prince Planet, the Prince does have superpowers and isn't invulnerable but merely able to dodge the Krankorians' weapons, which is a bit less embarrassing. Still, yeesh.
9) Lord Zedd and Rita Repulsa, Power Rangers
Has there ever been a more pathetic pair of alien invaders than these two Power Rangers antagonists? Despite an impressive amount of resources, these two basically limit themselves to sending one giant monster to Earth at a time, and usually let them run around while human-sized for a while first, only eventually making them gigantic so they can do some real damage. Maybe the send a couple of monsters down together, but if either of them had sent 26 monsters at one time, their invasion would probably have a higher chance of success. And it's not like the Power Rangers are the brightest of Earth's defenders, after all. None of the other main Power Rangers villains have faired any better over the years, but none tried as many times as Zedd and Rita, and their victory rate is pretty much stuck at zero.
10) Invader Zim
Invader Zim is supposed to invade Earth, hence his name. Unfortunately, his incompetence far outweighs his passion for such an endeavor, and thus all his plans are defeated by himself, his insane robot GIR, or a young boy named Dib who lives nearby and whom no one ever believes about these imminent alien plots to conquer the world. But unlike most of the invaders on this list, at least when Zim fails, he usually fails creatively.
11) The Aliens of Signs
Let's ignore the fact that a major portion of this alien invasion seemed to focus on one small Pennsylvania farm house — clearly a tactical necessity — or that these aliens spent a lot of their time making crop circles for no apparent reason. All that is dumb, but nothing matches the ignorance of coming to a planet that is 70% water, attacking beings that are 60% water, and being vulnerable to water. "All right, guys, we've found our next target. It's planet made almost entirely of poison, and we call it PoisonWorld. Any questions?" "Yessir. What if we invaded PoisonWorld… while completely naked?" "I like the way you think, Steve."
12) The Kraals, Doctor Who
Okay, one more Doctor Who entry, just for kicks. The Kraal's planet is dying, so they've decided to head to Earth. They've made some fake robot humans and a replica of a small village on their planet as a training ground for their invasion, complete with robot dogs — a little OCD, but being prepared is always smart. The Kraal have captured an astronaut; their big plan is to send the brain-washed astronaut to earth with a tale of being trapped around Jupiter and having to drink his own urine (they're very adamant on including this detail), which will somehow distract all of humanity so they don't notice a bunch of meteorites hitting the real version of the small British village. Once there, the robot duplicates will spread a virus that will wipe out humanity. This is all vastly overcomplicated, and it should be no surprise the Doctor and Sarah Jane are able to thwart it on about eight different levels.
13) The Oni, Urusei Yatsura
The Oni are tiger-striped apparel-wearing, horned aliens who travel to Earth to conquer it. Despite their overwhelming technological superiority, they give Earth a chance to avoid their fate — if someone can catch their leader's daughter, Lum, in a game of tag within one week. Lum can fly, so the Oni's odds are pretty good, but still, it's not a sound invasion policy to allow your opposition the chance to get a "get out of alien invasion free" card. It should also be noted that the leader of the Oni is named Mr. Invader, because Urusei Yatsura is awesome.
14) The Pods, The Body Snatchers
Not the classic scifi movie Invasion of the Body Snatchers, but the original book by Jack Finney, titled simply The Body Snatchers. Both versions include a race of pod people secretly replacing humans, but while the movie ends with a mystery as to whether the pod people will be caught or defeated, the book ends somewhat differently: the aliens give up when humanity shows a mild bit of resistance to the idea of being replaced. No pain no gain, pod people.