15 Reasons for Your Alien Invasion Story

If you’re writing an alien invasion story, the first thing to consider is why. Why are your aliens coming here? Space travel is difficult. The immense distances combined with the laws of physics means that getting to Earth would be incredibly expensive. Why would realistic aliens invest so much of their time and resources just to get to our corner of the galaxy? If you want your story to be believable, your aliens will need a pretty convincing reason to invade.

Dumb Alien Invasion Excuses:

Here are some reasons used in alien invasion stories and films that just don’t make sense.

• Would aliens come here for water? No, you can get water from countless uninhabited planets and asteroids. You just have to melt it.
• Mining ore? They would have countless asteroids full of iron, platinum, gold, and other resources in their own galactic neighborhood. No need to conquer anyone to get them.
• What about capturing people for forced labor? Not likely. Any species that has mastered intergalactic travel would also have plenty of robots to handle all its needs.
• Meat? Probably not. We’re starting to grow meat in labs right now. Advanced aliens could grow simulations of beef, pork, chicken, and even the meat of other sentient beings without the hassle of intergalactic war.
• Breeding? No, absolutely not. Contrary to conspiracy theories about The Grays, aliens would be unable to reproduce with us, and almost certainly unable to create genetically engineered hybrids. The aliens would likely have a different genetic structure, or not even have DNA at all.  

So what’s left? What are some more believable motivations for alien invasion?

Logical Motivation for an Alien Invasion:

1. Truly Rare Resources – Unlike gold, iron, and water, which are common throughout the universe, there are some incredibly rare resources found on Earth. If their planet has a technological or biological need for them that is strong enough, this may justify an alien invasion. Tantalum is the rarest stable metal, and is used in electronics, surgical instruments, and medical implants. Phosphorus is essential for life as we know it but is rare in the universe. An organism unique to Earth may also be valuable as a treatment for illness or disease, such as penicillium, a mold used to create the antibiotic penicillin, or bacteria like Bifidobacterium that help prevent stomach issues.   
2. Land – Yes, there are countless planets, but only a small percentage meets the narrow requirements for life as we know it. If their needs are even more specific than ours, or pressures from overpopulation are high enough, this may make an alien invasion worth the immense cost. Sure, any species advanced enough to develop intergalactic warships probably also has terraforming technology, but completely reshaping a planet’s environment is a long process. While you’re waiting for the right atmospheric conditions, billions of aliens are having billions of babies. Conquering an already habitable planet could be quite a bit faster.
3. Profit – The aliens may go around picking fights with other planets simply to create new markets for their intergalactic starships, fighter drones, orbital death rays, and other weapons. The planet’s governments start the wars while the planet’s arms dealers secretly sell weapons to both sides. The aliens may be convinced that they could sell Earth’s real estate for more than the cost of the journey. Or Earth artifacts or animals may be worth trillions to alien scientists or collectors.
4. Strategy in a Larger War – The alien invasion may be a small part of a larger war where Earth is just a staging ground. Aliens from Trappist declare war against Centauri, a planet about four lightyears from Earth. The Trappists are much, much farther away. They can’t afford to travel forty lightyears back and forth every time they need supplies, so they decide to conquer Earth and fight the battle from there. Earth could also be of strategic importance to aliens wanting to control intergalactic trade routes or hyperspace lanes.
5. They’re Pawns in Someone Else’s Game – The alien invasion may be part of a proxy war. A more advanced species could fund their invasions and use them to wipe out “troublesome” planets without having to get their own hands dirty. The aliens may be seeking membership in a Galactic Coalition and be required to conquer a planet to prove themselves worthy. They may even have been conscripted by the Galactic Empire, forced to invade Earth to prevent the Empire from wiping out their home world.
6. Space Piracy – The alien invasion is actually a robbery. The alien have stolen another culture’s ships and weapons, and are actually less technologically advanced than Humans. They are traveling the galaxy stealing all the tech that they couldn’t develop on their own.
7. It’s Just a Pit Stop – While Earth may be too distant to be worth the journey, what if the aliens weren’t actually trying to come here? The aliens could be on their way somewhere else, but run out of food, fuel, or other resources. They are forced to invade Earth so they can restock and continue their journey elsewhere.
8. Simply Massive Resource Requirements – The aliens may be involved in a construction project that requires a massive amount of resources, such as constructing a Dyson sphere to completely enclose a sun. While ore is common, a project this large would require all the ore from millions of planets and/or asteroids. Earth could be a relatively unimportant part of a project that destroyed countless other worlds.
9. Religion – The aliens may have religious beliefs that compel them to travel the universe “righting the wrongs” of other cultures. The aliens may find Human behavior offensive and wish to “correct” it, or to simply cleanse the universe of “sinners.” The aliens may believe that dying in war is the surest path to Space Heaven, so wiping out other species is actually the biggest blessing they could offer the universe.
10. Culture & Art – They found the Voyager record and fell in love with Chuck Berry’s “Johnny B. Goode.”  Obsessed with Human music, they headed to Earth for more. Just like Humans have looted the art of other cultures for centuries, the alien invasion is all about taking control of rock music.
11. They’re Malfunctioning – The distance between planets is vast, and as far as we know, you cannot travel faster than life. When we do encounter aliens, they may actually be robot probes sent by a species that died out eons ago. By the time the robot probes reach us, they may be malfunctioning and attacking anything that moves, stripping the galaxy of resources like a swarm of intergalactic locusts.
12. Altruism – The aliens may believe that Humanity will destroy itself with war, so the alien invasion may be intended to prevent that from happening. They may also believe that habitable planets are so valuable that any species that damages them should be wiped out. Or they could want to force Humanity to stop any of a thousand other evils, from factory farming to homelessness to our current political system. The aliens may even be pretending to invade, in the belief that uniting against an external threat will help Humans overcome their differences. Or the “invasion” could actually be a rescue mission. The aliens are invading to free Humanity from a hidden threat, such as an animal species that is far more intelligent than we realize, or shapeshifting aliens that have secretly conquered the planet.
13. Scientific Research – The alien invasion may be part of a long-term science experiment to study the development of consciousness. If they wipe out all intelligent life on a planet, which species will evolve sentience next? If a technologically primitive species is presented with an intergalactic threat, will their instinct for self-preservation accelerate their technological development? How do these weird, carbon-based lifeforms work, and how many of them do we need to cut open to figure it out? Or they may just collect Humans in the same way we might collect butterfly specimens to pin up in a display.  
14. Self-Preservation – The aliens may believe Humanity is too warlike, too religious, or too biologically dangerous (diseases, viruses, faster evolutionary speed) to be allowed to survive. They might decide to wipe out all life on Earth before Humanity can develop interstellar travel and do the same to them. Aliens might also invade Earth if they were forced to by another more advanced – and more dangerous – species.
15. Revenge – Humans are making our first steps out into the galaxy. Perhaps one of our space probes accidentally carried bacteria to an alien planet that wiped out half their species, sparking an alien invasion. Anger is a great motivator.

The most logical reason for an alien invasion may simply be none at all. The aliens could be too different from Humans for us to understand their reasoning.

They may be an essentially immortal species that no longer understands death, or a silicon-based species that does not consider carbon-based organisms to be truly alive, or a hive mind that cannot even comprehend what life is like for individual organisms. They may have vastly different sensory organs and brain structures, to the point that they perceive an entirely different version of reality. They may even be so physically large that Humans are simply beneath their notice. They may be multidimensional entities who are not even aware that their presence is wiping out other lifeforms.

Whatever reason you pick for your alien invasion story, it’s important to consider how your Humans are getting this information. How do they even know what the aliens’ motivations are? The aliens likely don’t speak any Earth languages. Any communications would have to be translated, and the translation would likely have inaccuracies. Any captured alien spies could be lying, or just too low-ranking to even know what the aliens’ plans are. Any captured computers would have to be reverse engineered, which could lead to even more errors.

When the Humans in your alien invasion story discover clues to the aliens’ motivations, keeping things vague and uncertain may make the most logical sense. It would certainly help avoid plot holes and logical inconsistencies. And you can always clear things up later in the sequels.

What do you think? What’s your favorite alien invasion motivation? Let me know in the comments!

Still need more ideas? Check out my story idea collection here, and my book Inspiration Overdose. And reading these space opera novels is sure to get your creative juices flowing. Enjoy!