This is a pretty common science fiction theme. It’s also a notoriously unforgiving one from a plausibility standpoint.
If you want to write really hard sci-fi, you might be well advised to stay away from the invading Martians.
That said though, there are ways to approach the concept that can help a lot, and I’m going to discuss them here. This article is, more or less, just a list of observations about the plausibility of different aspects of this trope. The intent is not that people should feel obliged to write scrupulously plausible Space Invaders, the intent is for people to pick and choose useful ideas.
People like alien invasion stories, and they are probably willing to give you more than the standard amount of leeway in writing about them. This will not stop them bitching about it if they feel that using a laptop to wipe out the invasion fleet was a step too far.
The following are the major sticking points for this type of plot, and all represent reasonable enough assumptions that you really need to specifically address them, and work hard, in order to go in a different direction.
What they have going for Them
Technological supremacy –
Any species that can get itself here, likely has a massive technological advantage over us. The harder your science, the bigger the advantage is probably going to be.
Technological supremacy is a really big deal.
There is a tendency, especially in film and television to depict advanced technology as regular tech with better special effects, that is to say, alien weapons for example, aren’t really shown as any more dangerous than a regular weapon.
Whilst the whole point about futuristic technology, for a writer, is that you can have it do pretty much whatever you want, you should be careful not to make it less effective than modern stuff, and be aware that the modern stuff is already much nastier than most people realize.
Tactical advantage –
Assuming that your alien invaders are starting out from space, and retain the ability to move around in space, then they already have a lot going for them.
They call it a gravity well for a reason, just picture two regular people, one of whom is at the bottom of a regular well, and one of whom is standing at the top, with a big pile of regular rocks, which of them are you going to put your money on?
Space is full of rocks.
It’s very difficult, probably impossible, for us to project force upwards into space at short notice, and it’s very easy for something else to project force downwards out of space.
A species that can get itself here can, and will, almost certainly have scouted us out first without being observed. At the very least they can learn a lot about us from television from a long way away. They have the opportunity to sabotage our infrastructure in advance and figure out in advance exactly how we will respond and where from. We are unlikely to have any information about them at all.
So, what might we have going for us?
The issue then is as follows, given the above assumptions, the aliens would tend to have an overwhelming advantage, especially if they just want to destroy all humans.
They can drop rocks on us, they can sabotage our infrastructure, they can nuke our cities’, or worse, and there is very little we can do about it.
If you want your human race to have a chance, you are probably going to have to do some work to give it to them.
There is no reason that an alien invasion needs to be an unstoppable juggernaut of destruction. We would have more of a chance if the invaders own resources are more limited.
It’s already likely that an invading alien force will be significantly outnumbered, perhaps overwhelmingly so. This will impair their ability to actually invade and hold territory.
Perhaps the aggressors only represent a very small opportunistic raiding force that needs to be beaten before they can leave and return in greater numbers?
Travel capability –
they might have precisely calculated their fuel usage to get here, or have used a means of travel that does not allow them to move effectively over short distances. Either would prevent them exploiting the advantages that their presence in space would normally give them.
They might be limited in the number of troops that they can move around at the same time.
They may be pushed to act immediately, their arrival may be impossible to conceal, or they might slip up and reveal themselves early.
Their spacecraft could be becoming uninhabitable, and they might have no choice but to land and attempt to establish a foothold immediately, or they might not use spacecraft at all, arriving directly on the surface.
Time constraints are a great way of preventing the aliens making effective use of some of the advantages that they should logically have, or of provoking a conflict that might otherwise be resolved with diplomacy.
They might not have expected a fight at all; or they may be acting on a target of opportunity and pushing their capabilities. It may become vitally important to prevent them from building infrastructure that can construct more weapons.
They may have lots of weapons but have only limited supplies of ammunition, and replacement parts, for them.
Aliens are unlikely to behave the same way that humans might, it’s possible that might give us an unexpected advantage or an oppurtunity that we could exploit.
They don’t want to kill us –
At least not straight away. They may want to minimize loss of life, or even avoid it completely.
They don’t want to kill all of us –
They might wish to spare specific individuals such as children or members of a given professions. They might treat individual societies on earth differently, either as a result of negotiation, or according to their own values and views of human society.
They may offer people the ability to choose not to resist on an individual basis, or actually intend to spare collaborators.
They are testing us –
They want to see our response or test our capabilities. In these circumstances it’s possible that we could fail by “winning”, as they may intend to wipe out species that attain a specific level of technology or capability.
They may be looking for evidence of aggression, or its absence. They may want to see how ruthless we can be.
They may wish to see whether we will behave honorably and keep to agreements.
They are manipulating us –
Perhaps they are manufacturing a threat in order to make us work together or induce us to prepare defenses against a secondary threat. Maybe they need to provoke us into some action to justify additional action against us. Maybe they believe in advancement through adversity.
They may wish reduce our population, perhaps in a very specific way.
They are indifferent to us –
They may have a goal that they intend to pursue with as little interaction with us as possible. Their goal may be positive or negative for us, or simply inscrutable.
They have no choice –
They may not wish to harm us, but they may not feel compelled into action by circumstances beyond their control. Some of them may be willing to offer us aid.
They want a challenging fight –
They are deliberately restricting their capabilities, probably not to the point of fairness. They may be testing their own capabilities, or being punished. They may intend to die fighting. They may wish us to appear more threatening than we actually are.
There are multiple factions or species involved–
They may be constrained in their actions to avoid reprisals or punishment from an external authority.
They may be fighting against another species, possibly one that would be willing to aid us, or who entered the fight as a third party. They may be fighting over us.
In all of these cases, picking the right side to support might be very important and difficult to figure out (although this is also something of a cliché, and difficult to set up without the resulting twist being very obvious)
They are inexperienced at fighting, or otherwise hampered in combat –
Maybe they come from a race that has no history of fighting amongst themselves, this could reduce the extent that their technological supremacy is actually reflected in the capabilities of their weapons. Maybe they aren’t very ruthless, or particularly committed to their objective.
Perhaps they value their own safety to the extent that it interferes with their effectiveness. Maybe they are unable to anticipate our actions.
They are mindless –
Perhaps they represent some kind of natural space dwelling organism attacking from space, possibly they represent an artificial entity of some type.
They miscalculated –
Perhaps they didn’t expect the planet to be populated at all. Maybe they are dealing with additional problems that we are unaware of.
Maybe they lost weapons, resources, and troops in an accident on their way here, perhaps their leader died of natural causes or in an accident.
Not just their primary objective, but any sub-objectives or priorities that they have that may inhibit their ability to deploy their full capacity against us.
There are a lot of resources in space, which should be easier to get at than planet bound resources, however, human civilization may provide opportunities for raiding aliens to target artificial concentrations of materials. Nuclear power facilities could be targeted for purified fissionable material, or Fort Knox for its gold. This could be especially credible if the invaders need the material at short notice, to fix their ship, say.
Human habitations also represent large concentrations of refined materials, so you might want to explore the potential of aliens intending to literally mine our cities.
Remember that moving materials out of a gravity well is a lot of work, so you probably want to concentrate on smaller amounts of rare materials.
Stealing resources makes less sense as a long term goal, and if they intend to gather their own resources then you need to explain why they are doing so on earth, rather than on some random uninhabited world.
Lifeforms and biodiveristy? –
It likely that the earths ecosystem, as well as underlying biochemistry, would be of tremendous interest to an Alien species, and it’s also possible that an alien race might want to at least preserve samples of things that they destroy.
They wouldn’t need large quantities of material to study, but such research could plausibly be time consuming, and require them to leave ecosystems as intact as possible, until they are finished.
This is not a consideration that is likely to stop aliens bombing our cities to rubble, but it might dissuade them from irradiating entire continents or using indiscriminate biological or chemical weapons.
The “invading earth to protect the planet from humanity” plotline isn’t really credible for a variety of reasons, but, more importantly, it’s been very overexposed.
Whilst their own technology is likely to be much advanced over our own, it is quite possible that we’d know at least few things that they didn’t.
The problem here, is that we have already made our knowledge very readily accessible. Much of what we know is either freely available or weakly protected and connected to the internet somewhere. We certainly have some secrets that aren’t plugged into a network, but these only represent a tiny proportion of our most significant knowledge from an outsider’s perspective.
This could be important if the invaders didn’t have any time in advance to research us. If they didn’t have the opportunity to access, decode, and download phenomenal amounts of information in advance, then any large scale destruction from them could easily disrupt the opportunity for them to easily take our information in the future.
Stopping them accessing this information could become a short term priority, although, realistically, it would make much more sense for us to do this by pulling information offline, than by trying to fight a “cyber war” with alien invaders.
The chances of alien invaders being able to eat Earth’s life forms, let alone justify invading an entire planet for food, makes this rather unlikely. It’s vaguely possible that a sadistic alien race might enjoy eating alien or even intelligent life, as some kind of delicacy (possibly after altering the organism to be more compatible with their biochemistry), but this only really makes sense as a side plot.
Life forms on earth do represent a concentration of complicated organic compounds that could plausibly attract unintelligent life that had evolved to spread through space and be very flexible in what it eats, but intelligent life is likely to be able to find or make its own food much more easily.
It’s unlikely that there is anything intrinsically special about humanity (on more than purely a biological and biochemical level) that would make us useful to advanced alien life, simply because they are likely advanced enough to create their own lifeforms, potentially incorporating any biological advantages that we did possess.
It’s certainly possible that an expansionist alien race would wish to conquer the planet without killing its population. It’s also possible that a race might invade to stop us annihilating ourselves or to steer our development in a specific way. This all requires at least some minor degree of benevolence on the part of the invaders however, as it’s difficult to justify them gaining anything special by sparing us in the long term.
Of course, it’s quite possible that they might be motivated to invade simply because they want to enslave or torture us for fun.
Our Brains? –
This B-movie staple is actually a fairly credible objective. If an alien race wants to examine our thought processes, record or archive our consciousness, upload us to a virtual environment, or even package us neatly for transport, then taking just our brains makes sense.
This is somewhat lazily evoked in a lot of fiction. The Aliens “don’t want to destroy our infrastructure”. If you are going to use this as an excuse, it’s probably sensible to consider what exactly the aliens want to preserve and why.
There is actually very little of our existing infrastructure that would be of much use to invading aliens with radically different technology, aside from the (xeno)anthropological perspective.
Our infrastructure is also fairly resistant to a lot of things that can be expected to kill us, such as radiation, and chemical or biological weapons.
A base of operations? –
This makes more sense if the aliens are able to breathe our atmosphere and can tolerate similar gravity, radiation and temperatures, as the presence of a hospitable environment is one of the main advantages provided by a planetary location.
The ocean could be an attractive environment for some alien life, and the presence of large quantities of accessible oxygen or water might be convenient, even if they do not intend to take them in large quantities. If they do just want a base of operations rather than a conquered planet then they may be amenable to negotiating a compromise or just trying to ignore us.
How things might play out
So, if the Aliens invade tomorrow, what is likely to happen?
Killing all humans –
If the aliens intend to wipe us out completely, then they are likely to deploy biological or chemical weapons against us, or simply bombard out cities directly. There isn’t that much in our cities that would justify putting troops down into urban centers, even if they are trying to preserve them for future examination, in fact, especially if they want to preserve them.
Fleeing into the countryside will only save people if the aliens don’t intend to be thorough, or hang about for long, and, regardless, the rural area in most nations would only be able to sustain a tiny proportion of their total population. If the invaders wish to preserve infrastructure they can use EMP devices and watch millions of people starve.
It’s possible that they might send ground forces to hit specific targets, but there is little incentive for them to fight us on the ground until they have thoroughly softened us up, no matter how overwhelming their military technology is. If they have any significant intelligence about us at all you can expect military bases and nuclear weapons to be priority targets.
Surrender or die –
In the event they present a demand we surrender, the more gung-ho amongst you might be disappointed. The military advisors of most countries will probably advise that we comply (there is evidence that the US, at least, has significant war planning for this) and your leaders will most likely follow that advice.
If any surrender is accepted then you can expect the aliens to move quickly to secure our leaders, disarm our troops, and establish complete control over telecommunications.
What happens next is probably out of our control.
Spamming the nuke button –
Assuming that the military situation doesn’t seem utterly hopeless (a single ship crashed in the desert, say), and the risk of escalating the situation isn’t considered too great, then you can probably expect nuclear weapons to represent our best hope of actually pulling off a military victory. The contamination potential from a well-executed nuclear strike is actually a very minor consideration when measured against the overall seriousness of the situation.
Regardless, any chance of a military victory requires the possibility of attacking the invasions headquarters or staging point, if this happens to be in space we are probably out of luck. It’s theoretically possible that we could target nearby space targets with missiles or somehow smuggle a nuclear bomb up, so you can expect any half way competent space invader to take steps to guard against that.
What happens next?
Personally I think that aftermath of an alien invasion has a lot more potential for interesting fiction than the preamble, I also think that aspects of this haven’t been explored that much, especially in visual media.
In the unlikely event that we have pulled of a victory, then we are still likely in a lot of trouble. It’s reasonable to expect more aliens to show up at some point, and they are likely to be angrier and better prepared for the second round.
Priority is obviously going to be rebuilding, as well as trying to reverse engineer as much alien technology as possible and attempting to fortify the planet against the next wave. Even if we can reduce the technological gap, defending a planet is a very difficult, possibly hopeless tactical problem. We may be better off trying to use captured technology to flee into space before they come back.
Expect some interesting political ramifications, especially if the initial attack created any power vacuums.
Forget scrappy rebellions, if we can’t defeat the aliens with all of our military and resources intact, we aren’t going to do it when we are firmly under their thumb, barring some mystic maguffin or secret weakness and an improbable opportunity to simultaneously wipe out all the invaders simultaneously.
Life on a planet ruled by aliens who aren’t secretly plotting to render us down into food or use us to gestate their eggs (and at this point, why would they need to lie about that) is rather under explored and lends itself to an awful lot of different types of story.
In the event of an alien invasion we’re probably doomed, but that doesn’t necessarily mean that your characters have to be.
I hope that something in this article is useful to anyone writing about this type of story, if there is anything in this article that people want explored in more detail than don’t hesitate to let me know about it in the comments.