Easily-Thwarted Alien Invasion

""You guys couldn't take over a damn bowl of Jell-O!""

- Frylock on The Plutonians, Aqua Teen Hunger Force

The easiest way to write an Alien Invasion is to make the invaders nigh unstoppable by giving them technology and weapons we can't even comprehend, defend against or overpower. If they were just like a human army, it might as well be a war story; there must be something about the aliens that makes them distinct from any other army.

But the heroes and humanity can't just be dispassionately crushed under a cosmic steamroller, because a Cosmic Horror Story would be a box office dud due to the bleak narrative and Downer Ending. Besides, who would want to read that kind of story?

So what authors do is craft an Easily Thwarted Alien Invasion. To compensate for the aliens' unstoppable power they build in a glaring and common weakness that the heroes and humanity can use to beat back this alien menace once it's identified.

This doesn't just apply to armies, but can be done for individual aliens and non-sentient alien monsters. Maybe their skin is tough enough to stop bullets, or they're telepathic, or they have mind-control powers, or they have some sort of martial society and cannot comprehend "mercy", "surrender" or "peaceful coexistence". Whatever their advantage or power, it's always balanced by a vulnerability to an over the counter product.

And sometimes, that Achilles' Heel is so serious that the humans don't even have to figure it out—the planet takes care of them without any help from us at all. The flora and fauna will make them all die naturally without human intervention, be it from the common cold or water or daffodils. Sometimes, the aliens don't even know it's a weakness (or can't conceive of it as one) until they begin dropping like flies from it.

This can extend to making the cleanup after the invasion a snap. By handing humanity victory on a silver platter and having an epilogue where the characters happily explain how easy it was to destroy the aliens and return to life as if nothing had happened. Grimmer portrayals will give humanity one chance to exploit the weakness after they devastate the planet, making the victory a Bittersweet Ending.

This trope can sometimes be justified if the "alien invasion" is actually only carried out by a few individuals, especially if they're completely ignorant about Earth, as is the case in many old Science Fiction movies.

Compare Rock Beats Laser. See also Weaksauce Weakness and How to Invade An Alien Planet.

Anime and Manga

 * Subverted in Super Dimension Fortress Macross, when just as the main cast finally deduce the aliens' weakness,
 * Keroro Gunsou; While mostly the characters are too lazy to pull off an invasion attempt, when they do you can bet it'll fall apart pretty easily.
 * One season finale involved a massive, monolithic alien the size of a city that was defeated
 * Urusei Yatsura has a few of these. The series started with the Oni deciding to invade earth, but then deciding to call it off if a randomly chosen earthling could beat Princess Lum in a game of Tag. Naturally, Ataru manages to win (even if he does end up unintentionally proposing to their princess in the process). Other invaders include a bunch in a spaceship that got mistaken for a rugby ball (the battering it took, plus the way Ataru chased the princess in charge after being miniaturised into it scared them off). Also there was the time a shapechanging alien came to earth and tried brainwashing the locals... unfortunately he disguised himself as Ataru... and his brainwashing technique involved pressing his lips against his victim's.
 * Heroman.

Comic Books

 * Toyed with in Blue Beetle, with the Reach, a race of invaders who know they're easily thwarted... so they come on like friendlies and engage some very long-term planning that will give them the planet in a couple of hundred years. When their plans are exposed, Earth's governments (DC Earth being no stranger to alien invasions) immediately demand the Reach's surrender. They get it.
 * In JLA: Year One the ruler of a planet of elemental giants sends seven of his greatest warriors to a certain backwater planet for a grand battle to decide the new leader. The battle never happens; all seven are defeated by local superheroes within minutes of their arrival. The warlord decides that Earthlings are too dangerous to live and orders a full-scale invasion. And is of course, beaten.
 * Of all the aliens in the Marvel Universe, the easily thwarted award has to go to the shapeshifting Skrulls. Their first appearance in Fantastic Four #2 sees Reed Richards tricking them into thinking Earth is too dangerous to invade by showing them drawings of monsters. In Secret Invasion, which sees the Skrulls throwing hundreds of advanced warships, dozens of overpowered Super Skrulls, and a new technique for completely undetectable shapeshifting, they ultimately lose after
 * Similarly, in an early comic, an alien lands on Earth with the intention of conquering it, but just happens to run into Thor, who promptly defeats him in battle. Thinking Thor is a native of Earth, the alien falsely belives that the entire human race is as powerful as him and he flees.
 * Played with with the Vespa in Irredeemable. At first, it seems that one lone Badass Normal hero snuck into their ship and forced them all to leave earth just when things looked grimmest.

Film

 * In the classic horror movie The Day of the Triffids, the eponymous carnivorous plants are killed off by sea water.
 * Contrast with the original novel in which the invasion isn't thwarted, the survivors instead having to adapt to the Cozy Catastrophe.
 * Independence Day. A fleet of scientifically advanced homicidal aliens has its force field defense taken out by a computer virus uploaded from a Mac, allowing the humans to blow up their base ships by attacking the ship's primary weapons as they fire.
 * Justified by the aliens' reliance on their innate telepathy resulting in their telecommunications technology being comparatively less advanced (hinted at in the opening scenes where the aliens use humanity's communication satellites to coordinate their attack, rather than their own ships, suggesting that they in fact did not have an equivalent capability on their own). The character in question tested his virus and his laptop on the recovered alien ship first, suggesting that either he or the scientists had already hacked the alien OS. Either that or human computer programming was inspired by leaked alien tech.
 * Also, it's a safe bet that none of their prior victims had the opportunity to reverse-engineer their technology and use it against them. (If they had, the events of the movie probably wouldn't have happened.) These aliens rely on shock-and-awe tactics to eliminate the major threats before they can respond.
 * The More Than Mind Controlling viral aliens in The Invasion had, in a matter of three or four days, infiltrated most of the Washington population and (going by the Shuttle debris pattern) must have had agents all over the US and the world, painting a scenario where humanity will inevitably fall no matter what it does. The film concludes with a near Reset Button, one character points out that we might never find and cure all infected individuals, but notes that we are in control of the planet because there are still wars.
 * Humorous example: Mars Attacks!! and yodeling.
 * Signs with water. Although it's not really necessary, given that this is a race that can be stopped by a pantry door that probably wouldn't have kept a determined human in. Seriously. It's like if humanity invaded the acid-covered world, with acid rain, and invaded the natives by wandering naked into the rural farmhouses.
 * Evolution had the aliens thwarted by Selenium Sulfide.
 * Prince of Space. The chickenmen of Krankor are able to travel half a million miles to invade Earth, even though their fuel capabilities lag far behind ours, where they have their asses handed to them repeatedly by a man who has no superpowers, but can skip reasonably well. Turns out that their weapons have no effect on him.
 * Isn't "invulnerable" a super power?
 * The dissonance comes from the fact that in the original he wasn't immune to their weapons, but was dodging them. In the dub he claimed that their weapons couldn't hurt him - but still dodged when they shot at him.
 * Aliens in the Attic runs on this.

Literature
""For thousands of years the mighty starships tore across the empty wastes of space and finally dived screaming on to the planet Earth - where, due to a terrible miscalculation of scale, the entire battle fleet was accidentally swallowed by a small dog.""
 * Harry Turtledove's short story "The Road Not Taken" posits that the secret of interstellar travel is an absurdly simple technological concept (so much so that it seems obvious in retrospect, like the wheel), and yet Earth, by sheer happenstance, never stumbles upon it. Later, Earth is invaded by aliens in wooden spaceships armed with stone-tipped spears cannons and black powder muskets... who are confronted by humans who, having never discovered FTL drives, have instead devoted their research to other scientific pursuits, such as weapons that outclass the invaders' by centuries of development. The story ends with the captive aliens horrified that the humans will be able to discover the secret of hyperdrive from their ship, unleashing the violent, tremendously advanced (compared to other species) humans upon the rest of the galaxy.
 * The sequel picks up a thousand years later, when the human race (now stagnant for the same reasons as everyone else) tries to invade a race that still hadn't found the trick, but were now advanced enough that they don't need it to carve out an interstellar empire.
 * In the Worldwar series by Harry Turtledove, the Race, lizardlike alien invaders, intend to avoid this by showing up loaded for the bear. Their probe showed mounted knights on horseback, spear wielding tribesmen, and wooden ships. Thus, they brought automatic rifles, tanks, jet fighters, and nuclear weapons. The time between their probe sending them the information and the invasion force arriving was 800 years, but things don't move on much in that kind of time, at least not for the Race and the other planet they've conquered. Unfortunately for them, "Tosev 3" is different. The invaders show up in the middle of World War Two to find humans not able to match them one for one, but certainly able to compete with them and defeat them, especially when the humans vastly superior numbers and willingness to sacrifice themselves in droves come into play. It Got Worse when winter rolled around, and the Race realized that, compared to their home planet, Earth is REALLY cold during winter. Especially in Russia. It turned an expected walkover into the aliens having to be content with conquering only parts of the planet that couldn't fight back effectively, or were in the equatorial belt and warm enough for continuous operations.
 * It's debatable whether you could really call this "Easily Thwarted", though. After a series of Balance novels, the Race is fought to a standstill, and the Colonisation novels open thirty-four years later, with most of the Southern Hemisphere under their control.
 * Technically, in last novel, Homeward Bound, humanity already outpaced the Race technologically, and while they have numbers on their side (for now), in a century humanity will out-pace them to a point where competition will be impossible.
 * It's also mentioned that the reason humanity had a fighting chance was the fact that World War II was going on and that technology hadn't reach the point where the Race's EMP would have done considerable damage.
 * The premise for the Dawn of Victory mod for Sins of a Solar Empire is loosely based on this novel. The Scinfaxi (not their real name) are a vast interstellar empire, who set their sights on Earth. Same deal, their invasion force arrives centuries after the initial probe, and they land in the middle of WW 2. However, their tech is more akin to the aliens from War of the Worlds. Despite being shocked at humanity's rapid technological progress (Scinfaxi are conservatives when in comes to progress), they manage to conquer most of the world before the Russians manage to steal one of their nukes and blow it up. The Germans and the US follow suit, scaring the aliens into retreating to the Southen Hemisphere. A century later, humans abandon Earth and nuke the hell out of it, to kill any earthbound Scinfaxi. Thus begins a war between the aliens, Soviets, Germans, and Americans... In Space!
 * Subverted in "When The Tripods Came", the Tripods prequel. The first three Tripods are easily destroyed, and everyone believes this trope. But then The Trippy Show comes along, and...
 * Out of the Dark by David Weber goes along a similar premise to World War except that it takes place in the modern day and humanity is pretty much screwed,
 * Anne McCaffrey abuses this trope.
 * In the Freedom series, the preferred host species of the Eosi (body-snatching Evil Overlords) is deathly allergic to a plant that grows in abundance on Botany, the planet the main characters were stranded on. La Résistance manages to sneak boatloads of the plant's pollen into the ventilation system at a suspiciously fortunate gathering of 90% of the Eosi. The surviving Eosi were too few and scattered to retain their grip on their servant races.
 * In the Acorna series, the Khleevi (Exclusively Evil planet despoiling insectoids) are easily cowed by one squadron of battleships fighting back (though they were portrayed as bullies who preferred easy targets). And, after that, the heroes find a plant whose sap acted like purest acid on Khleevi exoskeletons.
 * In the Talents series, the Hivers (a more rapacious version of the Buggers from Ender's Game) prove to be completely unprepared for human telekinesis. Later, humanity found a pheromone combination that turned the Hivers from Borg-like aggressive conquerors to more docile, agrarian types.
 * In the Dragonriders of Pern books, Thread is a ravenous mycorrhizoid spore that can eat virtually anything carbon-based and burrow into the earth. But water (even a good soaking thunderstorm) kills it.
 * So does fire.
 * And cold. In one book, the Pernese don't realize that Threadfall has started because the first few falls are over the northern area of the continent during winter.
 * Or
 * If it seems improbable that Thread could ever live on any planet with anything worth eating,
 * Battlefield Earth had a very specific set of weakness that made their occupation of Earth (not the invasion itself) easily thwarted... as well as the annihilation of their race. The gas the Psychlo breathe combusts in proximity to radioactive materials, but no alien race fighting them was ever in a position to best them militarily and get a nuke near them (only a nuclear device could combust an atmosphere's worth of gas). As it turns out their teleporter net connected all of their planetary holdings everywhere, so if a nuclear device were smuggled on their homeworld and the detonation was timed to coincide with these teleporters activating it would incinerate every Psychlo in the universe. Luckily for them, no aliens could get near their conquered planets, and none had the intimate knowledge of Psychlo technology and organization to pull it off... until one of them gave a human the smarts to figure it out.
 * In a straighter application of this trope, Johny is inspired not to think of the Psychlos as invincible when Terl shows him the site of greatest military resistance... the US Air Force Academy. The cadets had figured out the Psychlos were using nerve gas and wore gas masks, and proceeded to put up quite a fight until overwhelmed by numbers.
 * Granted, "quite a fight" is sort of a misnomer. They conquered Earth (according to the movie) in three minutes. So this "quite a fight" must have been all of two minutes long. Not to mention the fact they annihilated our military in three minutes implies they had hella fast deployment capabilities... which, if you think about it, makes sense as they could probably use teleporters to just beam down a few tens of millions of dudes.
 * The Psychlos actually put a radiation detector on their transporter with it programmed to not transport anything radioactive. When the nukes were sent in the teleporter detected radioactive material and activated a quarantine shield; the issue was the humans kept teleporting in nukes, and the planet had been so thoroughly mined for resources, with so many holes left over from mining, that the blasts reached the core of the planet and (somehow) made the core react explosively, destroying the entire planet.
 * In the proper application of this trope, when the humans see the airplanes (Which had been previously ineffective), they are able to easily learn how to use them and fight the Psychlos succesfully. Despite having learned how to use these things in a matter of weeks and them not having worked the last time.
 * The reason that wouldn't work is because...well, because it wouldn't work. The movie was as bad in comparison to the book as the Eragon movie was to ITS book. They just had to make it seem like humans were awesome. In the actual book, they take advantage of the alien's superiority complex and greediness to take out or capture almost all the aliens on the planet at the same time, as well as teleport a bunch of nukes to their home world, the nukes being HIDDEN IN LEAD COFFINS. And even then it didn't work! The Genre Savvy aliens had thought of that, and had put up a super strong Force Field over the teleportation platform at the receiving end, hooked up to another radiation detector...too bad for them, apparently they needed to have a power source inside the force field, allowing the "planet buster" nukes to burrow down into the super-large planet(already riddled with tunnels from ancient digging), explode, and cause a continuous nuclear reaction that turns the entire planet into a sun. So, not so easily thwarted, only mildly easily thwarted. If the nukes weren't apparently completely invincible(They go off one after another, with apparently no damage from being next to 500 megaton nukes confined into a 50 foot area?), the psychlos would have survived just fine.
 * In Arthur C. Clarke's Space Odyssey series, the godlike aliens who blow up planets and mess with humanity's evolution from apes are found to be vulnerable to computer viruses... the code for which are buried in a mountain on the moon.
 * However, the supercomputer that the virus is uploaded to
 * Also, it is explicitly explained that the viruses in question are not just your regular run of the mill viruses - they exploit the laws of mathematics such that ANY computing system that uses numbers is vulnerable, no matter how advanced, and NO countermeasures of ANY kind are possible.
 * So anyone with a reasonable level of mathematical ability is doomed? But seriously, an attack against mathematics itself is clearly capable of killing any civilisation worthy of the name stone dead. Its like Kryptonite for sentience, and even the fact that such a thing could exist could only be the result of a universe created by a particularly malicious being.
 * The supervirus were described as working by meaning that they could in theory work against any calculating system no matter how advanced. However they presumably can be stopped at the point of infection by a good firewall like any other virus, which was probably how they were in the end contained and put into the lunar vault with all the other weapons of mass destruction. The target was the monolith, which was just a single machine, a single mostly autonomous tool of the aliens, not the alien civilization itself, and in fact  which may have made it more vulnerable. But even then humanity would have had no hope in hell of successfully infecting it at all without  . The conclusion of the last novel also left it ambiguous as to whether the supervirus actually worked or not, or was even necessary. (The monolith  and humanity was just being overly paranoid. The supervirus might have failed completely and the monolith  because that was what it was intending to/instructed to do all along. Alternately the monolith was destroyed by the supervirus, but fights it off long enough to send a You Suck message to humanity, and call for reinforcements, which presumably would arrive sometime around 5001.)
 * So the virus works by
 * Older Than Radio: The War of the Worlds did this with bacteria. In fact, it's pretty much the Trope Maker and Trope Codifier for the "The planet takes care of the invaders itself" version of this trope.
 * The novel implies that the Martians were severe germaphobes who rid their native habitat of all microbial life, thus blessing themselves with major suck right before the invasion of Earth.
 * In Wells' defense, he was writing before the discovery of antibiotics. Prior to WWII, far more soldiers of any invasion died of contagious diseases than battlefield injuries, and even in peacetime the British Empire lost scads of troops to malaria, yellow fever, and other tropical ailments in its overseas possessions. So the Martian imperialists weren't suffering from anything that Europe's own imperialists hadn't. (Wells took a dim view of imperialistic expansion.)
 * Subverted in The League of Extraordinary Gentlemen: Book II, where the bacteria which kill the Martians are actually.
 * The Hitchhiker's Guide to the Galaxy:


 * The trope is majorly inverted with the case of the Golgafrinchan Arks, where the Golgafrinchan refugees are Too Dumb to Live and yet crowd out the native neanderthals - becoming the ancestors of modern mankind as a result.
 * Robin Cook's Invasion had the Body Snatcher-esque aliens are thwarted by the release of a rhinovirus aka The Common Cold.
 * Keith Laumer wrote a hilarious short story in which aliens invade a dying civilization—only to discover that the locals were in fact immortals whose metabolisms shut down temporarily if they didn't get enough of a certain gas in their air. Guess what the invaders exhaled. And the locals who were still up and about were the weakest of their species, the scrawny ones who didn't need much of the gas. The invasion revived all of their brawny Badasses.
 * The invasion of the Gorg in The True Meaning of Smekday. For some reason, the Gorg try to kill every single cat on planet Earth. As it turns out, the Gorg are
 * The Body Snatchers, the short story upon which the film versions were written, had the aliens simply give up when the figure out they can't tolerate human resistance. The films tend to end on a darker note...
 * Rather than a single cataclysmic event, Christopher Anvil's short story "The Gentle Earth" covers a long campaign in which the invaders are slowly worn down, but they're defeated in such a thorough and humiliating fashion that it goes under this trope anyway. Their errors and catastrophes include but are not limited to failing to prepare appropriately for Earth's much colder winters, dismissing tornadoes as a legend (having landed in Tornado Alley), and drastically underestimating Russia's missile supply.
 * In Stephen Baxter's novel Space has the solar system invaded for use as an stellar fuel depot
 * Brutally averted in Greg Bear's Forge of God where humanity is totally screwed even before we realize we're under attack, have absolutely no way of even inconveniencing the invaders even a little bit even after we find out, and the total destruction of the earth is a foregone conclusion from essentially the first page. In the end we nuke a few decoys but never even see the aliens or their principle weapons.
 * In Mikhail Akhmanov's Invasion, the super-powerful alien starship wipes out a large chunk of Earth's fleet and shrugs off a nuclear Macross Missile Massacre without a scratch. In a Deus Ex Machina fashion, a different alien teleports onboard and gives the hero a device that destroys the ship's brain, shutting down all systems. In a brutal subersion of No Endor Holocaust, many major cities get hit with smaller falling alien ships full of antimatter, with the casualty count in the tens of millions, not to mention all the material damage. Earth recovers remarkably quickly in the sequel, though.
 * Lampshaded by the character of Zellaby in John Wyndham's The Midwich Cuckoos (1957), where he brings up a few examples of this trope common in British and American science fiction at the time in a strategy discussion. He then concludes the alien invasion they face is a war like any other, and takes the appropriate measures.
 * Sirens Of Titan by Kurt Vonnegut features a Martian attack on Earth.
 * Averted hard by the New Jedi Order series, in which it takes nineteen books spanning five years of war to defeat the Yuuzhan Vong. It's not until the New Republic/Galactic Alliance readopts asymmetric warfare strategies and combines them with Imperial tactics (like orbital bombardment) that the tide turns permanently.

Live Action TV

 * Lost in Space episode "Invaders From The Fifth Dimension" features Will Robinson abducted by a group of aliens who are repulsed by the sight of tears due to being unable to understand emotions, and eventually let him go as a result.
 * The TOS Outer Limits episode "Specimen: Unknown" had alien plants that release a deadly gas and spread like wildfire... literally. A spaceship carrying them crashes on Earth and it looks like the human race is doomed. Worse yet, a thunderstorm is brewing overhead, and the plants will spread even faster when they get some rain, right? Wrong. The rain causes them to shrivel up and die, and the world is saved. Think of it as the opposite of a Cruel Twist Ending.
 * Star Trek: The Original Series episode "Day of the Dove". An alien powerful enough to destroy a Klingon battlecruiser, transmute matter and control the minds of hundreds of beings simultaneously takes control of the Enterprise. How do the humans and Klingons get rid of it? By laughing at it. Seriously.
 * Most alien invasions in Power Rangers. Almost universally taken out by "teenagers with attitude". Also, supersuits and giant robots.
 * And not much in the way of attitude, when you get down to brass tacks. Nonetheless, five teens restricted to a single city with no military skill or training whatsoever fend off entire armies that know where they live, who they are, and what they like to do in their free time.
 * Better call Apeldoorn... or maybe not.
 * Doctor Who loves this trope, with dozens of aliens invasions ranging from single vessels to vast fleets (The World Is Always Doomed, after all) being Easily Thwarted by a single man with unusual dress sense. The spin-off series The Sarah Jane Adventures plays it even more straight than its source material.
 * This generally reaches its extreme whenever the Doctor fends of Dalek fleets. For reference, Daleks are the scourge of the universe, with precious few civilizations being able to stand against even small amounts of them.
 * The Science Fiction Sketch of Monty Python's Flying Circus, in which blancmanges from Andromeda seek to win Wimbledon by turning everyone in the world into a Scotsman. They were thwarted when two people ran out onto the tennis court and ate the blancmanges.

Video Games

 * Subverted hard in City of Heroes- the extradimensional Rikti invaders turned out to be vulnerable to magic, so they targeted magic users as soon as they found out. Their invasion still caused massive devastation, though not quite to After the End levels, and decimated the Superhero population.
 * And then they started learning how to use it themselves.
 * Averted entirely in Universe At War - The tutorial mission is the only time you play the humans and its Unwinnable; Earth has to be rescued by other aliens.
 * In the original X-COM, the aliens' weakness is logistics. They simply don't have enough forces available at the beginning of the game to launch more than a couple of small missions a month (though their forces build up more and more later), requiring them to operate in secret through infiltration and subversion of national governments, showing the world that X-Com is too weak to defeat them and attacking and destroying X-Com bases instead of open warfare. Meanwhile, even one crashed alien craft allows for humans to import their phlebotinum, and enough research allows them to.
 * The Rhombulans in Elite Beat Agents are so vulnerable to music that rhythmically beat garbage can lids is enough to knock out their soldiers. The easily-thwarted part happens when you remember what genre this game belongs in.
 * Half Life averts this hard. The scientists in the original game come up with two plans that they hope will Easily Thwart the invading Xenians, but the first is countered by the invaders, and the successful execution of the second plan actually has the opposite effect, and causes millions upon millions of Xen creatures to be teleported all over Earth. The best part? All of this interdimensional activity attracts the interest of a nigh-unstoppable alien empire known as the Combine, who then conquer Earth in an invasion known as the Seven Hour War.
 * The Shroobs in Mario & Luigi: Partners In Time. Right at the very end of the game, baby Luigi's crying on a Shroob Mushroom reveals that the Shroob materials dissolve when exposed to baby tears. The present-time E. Gadd creates an unmeasurable amount of water with the same composition as baby tears in response to this. He then pumps it through the time-holes, saturating the past Mushroom Kingdom in baby tears, completely removing any traces of the Shroobs from the Mushroom Kingdom.
 * In Earthbound Zero a troupe of three small children (albeit with Psychic Powers) defeats an all-powerful alien by
 * What about the sequel EarthBound where the said alien is defeated by ?
 * In both cases, it's more of a sort of softcore Mind Rape performed on Giygas than a Weaksauce Weakness, due to the circumstances.
 * Regardless of the tactics used to defeat Giygas himself, a group of 3 or 4 children basically are capable of fighting off everything Giygas can send at them in a more traditional sense.
 * Averted hard in the second Dawn of War 2 expansion. In the previous games, the heroes kill the alien Hive Tyrant, destroy their fleet, kill the ork warboss and even take out both the Grater Daemon and the Chaos Lord that tried to free it, so everything is okay, right? WRONG. Ten years later the war rages on with no end in sight and the situation is so grim that the Empire decides to just nuke the entire sub-sector. This is, naturally, par for course in Warhammer 40,000.

Web Comics

 * In The Inexplicable Adventures of Bob, everyman Bob has thwarted two alien attacks?one by Fructose Riboflavin and one by the Pirates of Ipecac?though in both instances, he had help.
 * Saturday Morning Breakfast Cereal: "Blast! Their iron is much stronger than our bronze!"

Western Animation
""In the end, the aliens were defeated not by guns or bombs, but by that humblest of Nature's creatures... the Tyrannosaurus Rex.""
 * Parodied in Futurama's version of The Twilight Zone, The Scary Door.

""The human was impervious to our most powerful magnetic fields, yet in the end he succumbed to a harmless sharpened stick!""
 * Another humorous example example, this time from a B-Movie on a planet of killer robots:


 * Fry repelling the Brain invasion by writing a story full of plot holes and spelling errors.
 * In The Simpsons, the childless child-hating adults, who had disposable income and free-time on their side, "had no protection against God's lowliest creatures... children." Or to be more precise, 'kid germs' to which they had no natural resistance.
 * In a Halloween special, after humans have destroyed all known weapons, Moe, acting alone, defeats the alien invaders with... a board with a nail in it.
 * It helped that the entire invasion force consisted of Kang and Kodos alone weilding a slingshot and a bat.
 * Another attempt was done when Kodos creates a portal which sends an army of aliens to Earth it was thwarted by the army which easily slaughters all the aliens.
 * And yet, in another episode they win because they use our stupidity against us. They kidnap and impersonate Clinton and Dole just before the 1996 election. When they are discovered they tell humanity that it doesn't matter since we have a two party system and we have to vote for one or the other.
 * In one episode of Care Bears: Adventures in Care-a-Lot, Care-a-Lot is invaded by Gobblebugs, tiny bugs that eat any and all plants, except ones that are yellow. Once the Care Bears realize this, they turn all the remaining plants yellow, and the bugs lose interest and leave.
 * Invader Zim: Not all the Irkens are idiots. Most of them (Especially Invaders) are actually pretty competant. It's just that the story is about Zim. An invader who is so stupid that they sent him into an uncharted area of space to find a planet they made up just so that he would die slowly and horribly due to starvation. They didn't anticipate Zim actually finding the planet.
 * This trope is also parodied in the episode "Germs", where an Easily-Thwarted Alien Invasion (by germs) in a bad sci-fi movie makes Zim paranoid about falling into the same pit-trap, and Hilarity Ensues.
 * It's generally implied, however, that even though the Irkens aren't always pillars of intelligence, they have a VERY powerful armada and are helped by most of the universe being even dumber.
 * However, it should be mentioned that most of the Irkens advanced technology is created and designed by another alien race, the Vortians, who were Invaded and conquered by the Irkens. For example, we find out they built and designed the Massive.
 * Agent Bishop faked up one of these to get extra funding for his organisation in the recent Teenage Mutant Ninja Turtles series.
 * Aqua Teen Hunger Force gives us the Moonenites, the Plutonians, the Jupiterians, Travis of the Cosmos, and the Frat Aliens.
 * In the Grand Finale of Kim Possible, two aliens and their robot army invade the world "in the time it takes to order a pizza", but only hold on to it for a few hours before being defeated.
 * That's not an example of this trope; the aliens were defeated only after the arrival of Mad Scientist Dr. Drakken's mind-controlled fast-growing Alien Kudzu and Kim's Sidekick Ron Stoppable's Super Mode turned his Badass Abnormal powers Up to Eleven... neither of which qualifies as a mundane Weaksauce Weakness.
 * Still, the heroes wouldn't have managed that in the first place if they hadn't brought down the Mother Ship by finding it's off switch.
 * Not a direct example, but in one episode of the original Transformers cartoon, the Decepticons ceased their attack on the Autobot base because the chaos caused activated it's fire-repellent systems... which was made up of fire-retardant foam being sprayed from the ceiling. According to Megatron, if they stayed and continued the attack, the foam would permanently damage their internal circuitry and they would shut-down for good. These are highly advanced Mechanical Lifeforms who have been shown surviving being frozen in ice for centuries and being submerged in the deepest areas of the sea and yet apparently the one thing most damaging to them could potentially be found in fire extinguishers. Imagine how easy it would be to get the Decepticons off planet if they just used those?
 * Remember, aim at the base and sweep.
 * In The Powerpuff Girls episode "Beat Your Greens", Earth is invaded by a race of broccoli aliens after all of the adults are brainwashed by eating modified broccoli. Naturally, the children escape this fate and fight back, by literally eating the invading aliens. Naturally, the aliens (and probably some viewers), were absolutely horrified by this.
 * Although the invasion is never shown to have actually happened, in Lilo and Stitch Earth avoids invasion by being the natural habitat of a highly endangered species... mosquitoes. What's more, the decision was apparently based entirely on the testimony of the Earth-native who tricked them, with not even a basic survey done to see if it was true.
 * There was a little bit of implied help from the seemingly Obstructive Bureaucrat, who in a twist of that trope was helping the good guys for once. At the very least,
 * In Big Guy and Rusty The Boy Robot, the alien Squillacci are the ones who draw crop circles and experiment on cattle. Their repeated invasion attempts are stopped only by a guy in a Humongous Mecha and an android the size and mentality of a child.
 * In Journey to Saturn, the alien invasion is thwarted by having the heroes throw and shoot beer at them. It probably wasn't a good idea to invade Denmark.
 * In the Birdman episode "Skon of Space", Skon, the advance scout for an alien invasion attacks Birdman. Birdman defeats him and sends him home with the warning that everyone on Earth has powers like him, so any invasion would be futile. Birdman calls Falcon 7 and warns him to prepare for an invasion just in case Skon's superiors are not as gullible as him.
 * Played with in Ben 10: Alien Force with the Hightbreeds, whose invasion turns out to be unstoppable in the finale when the protagonists attempt to thwart it by force. The only way Ben is able to stop the invasion is by finding out the reason behind the invasion and help them in a way so they wouldn't need to invade/destroy the universe anymore. Weither this count as easily thwarted or not is debatable.