Ever since H.G. Wells' War of the Worlds, the notion of extraterrestrial aliens attempting to conquer the Earth has been a popular SF topic. The recent television series Space: Above and Beyond and the movie Independence Day use this basic theme to allow for an all out struggle against am inhuman enemy, allowing for perhaps the only opportunity for a war story against a real Enemy in these post-Cold War days. But why and how would aliens try to take over our little world?
Let's look at "why" first. The aliens want to take the planet for some kind of reason; not necessarily a sane or comprehensible reason, but still a reason.
These aliens see our Earth as prime real estate with inconvenient inhabitants that can be dealt with, perhaps the way Europeans looked at the Americas. We have a reasonably habitable world with a lot of potential for food production and still a fair amount of mineral resources available. The inhabitants can be killed off, enslaved, or placed on reservations; then they start handing out the homesteads.
The problem with this idea is that there's likely a whole lot of worlds out there that don't have heavily armed inhabitants. If the aliens have an FTL (faster than light) drive they can almost certainly find a better choice, and would likely be able to scout the Earth first and determine that there would be problems in conquering our piece of rock.
If our aliens travel in generation ships or another slow form of transit they would probably first determine the presence of habitable worlds by advanced telescopes. They'd likely check for radio signals so that they'd not run into major competition. But with slow travelling space vessels it may take decades or centuries to arrive at the new colony world. Only a century ago, there'd have been no radio signals being broadcast into the cosmos from the Earth. Aliens that started out over one hundred years ago for our solar system with the assumption that Earth lacked an advanced race would be in for a major surprise.
Our aliens may have little choice but to still settle on Earth unless they have the means to build space colonies or settle on one of the lifeless worlds of our system. Possibly, our aliens would not be in a position to conquer and would be forced to trade their technology for living space, or would end up like the "Newcomers" in Alien Nation and attempt to live amongst us as a minority.
This would be an interesting, although perhaps difficult, role playing idea; members of an alien minority on contemporary Earth. A lot would depend on just how advanced the aliens were, what sort of deal they had to make to live on Earth, how humanlike they are, and how accepted they are by native Earthfolk.
Our Earth was once loaded with a treasure trove of minerals and valuable animals and plants. Even after millennia of exploitation, there's still a lot of valuable resources available. We Earthfolk certainly have fought wars of conquest for resources; extraterrestrials might do the same thing.
The problems I noted above for "living space" also apply here. Also note that mineral resources are readily available on other bodies in pretty much any star system, particularly on low gravity asteroids and in comets. One resource that aliens would almost certainly not be trying to acquire is water; while the "Visitors" in V were trying to steal our water in reality water is perhaps the most the most common compound in the universe and can readily be extracted from comets and icy moons and other bodies.
Products produced by plants and animals or from their buried remains might only be available on the Earth or other habitable planets. If some biologically produced substance is valuable enough, that might inspire a attempt to control the supply. This is a bit more of a reasonable reason for an invasion or other form of domination and control.
Our aliens may regard themselves as the Supreme Race with the Supreme Weapons and the Supreme Pizzas, to borrow from the MST3K showing of Teenagers From Outer Space. Such a race may consider Empire to be their Destiny, and would want to add the Earth to their Empire.
Such aliens may not be evil sorts; they may consider themselves to be the only race civilized enough to rule the Galaxy, or may simply be relatively nice to peoples who submit to their rule. On the other hand, they may consider their subjects to be their rightful thralls, to do with as they will.
Dreams of empire may not require other justifications. While there may be additional reasons to incorporate Earth into the Empire, the main reason for the conquest is the addition of more territory and another people as subjects to the Empire. Incorporation into a benevolent empire may actually be a positive thing for Earthfolk.
Our aliens may be conquering us for a religious reason. They might be wanting to convert us to their religion, or their religion mandates the acquisition of an empire, or they may see their religious teachings as suggesting that they must save us from our own self- destruction.
Such aliens may be the kindest beings in the galaxy, or the cruelest. It would depend on their god or gods; do they follow a god of war or a benevolent god who wants its followers to save others?
The silliest reason, immortalized in such films as Mars Needs Women, is a need for new mates due to reduced numbers leading to inbreeding or radiation causing sterility or whatever. The aliens land and grab women (or men, why not?) to take to their planet and force to bear (or father) their children. The aliens might also just want the women or men for concubines; while they may not be able to have children, the humans might still be useable for recreational sexual purposes. This doesn't necessarily require conquering the planet, but it might be helpful to do so.
Reasonably, humans and aliens can't just mate in the usual fashion and produce offspring unless they both have common ancestors and were separated by some means (Atlantean Starships, or Trek Preservers, or Traveller Ancients). There could be hybrids produced by high biotechnology. Octavia Butler's "Xenogenesis" series has a race of aliens that sets out to breed with other races, including humans; I've not read them so I don't know if the aliens use technology or some sort of peculiar adaptability.
One variation on this is an alien race that seeks out new members to join their race for some reason. Dr Who's "Cybermen" sought to perpetuate themselves by taking humans (and maybe others) and over time converting them into cybermen. Star Trek: the Next Generation's "Borg" pretty much do the same thing; "assimilating" other races into the Borg Collective.
There's a few stories where the aliens want to eat us. The lizard- like aliens in V liked to eat humans, and Niven's Kzintis like all kinds of meat. It's not likely that our aliens are going to conquer us for a food source, as there's better animals for food purposes than humans. They'd be eating us as an expression of their superiority, or as a gourmet treat, or just as part of reducing our population.
It's possible that our aliens, while using us for food, might not actually kill and eat us. An alien "vampire" might draw off some blood from its enslaved humans for a primary food source or a taste treat. They might consume other fluids or body products also.
A more fantastic variation on this is an alien that eats "life force" or "psychic energy" or some such thing. This may not be fatal or even harmful to the "donors"; it may even be pleasurable. Picture an alien feeding on energies from a retinue of pampered humans; they get a better lifestyle than their fellow conquered humans and perhaps even derive a (maybe sexual?) pleasure from their patron's feeding.
Some aliens may want our world's trade. We're an untapped market with unique products as far as the rest of the galaxy is concerned. We might also be a cheap labor market; after all, we're probably relative primitives who would be willing to work our behinds off for the equivalent of glass beads and cheap trinkets.
It's possible that the aliens would attempt to dominate us through a show of military force and establish a "protectorate"; they'd tell us that they are bringing us into galactic society while protecting us from hostile forces, but might really be exploiting us naive savages. They'd not likely do much of an invasion; they could establish their claim to our system as far as other powers are concerned by establishing some kind of permanent presence. They may just establish their base on the Moon or elsewhere and then announce to the Earth's governments that the Earth was now their protectorate and any contact, commercial or otherwise, with extraterrestrials had to go through them.
A moral alien race might negotiate with us to use other bodies and resources of our system that they could just take. A race wanting to set up colonies in the asteroid belt might "buy" some asteroids and pay for them with technology, and then establish trade with us for other things. See Asimov's "Buy Jupiter" for an example of a similar trade.
From our standpoint, we might experience sudden wealth and technological progress. We might not realize or care that from the perspective of the aliens we might be barely civilized barbarians willing to give up things of value for junk.
It's likely that there is more than one intelligent species of extraterrestrials; there are probably lots of them. It is conceivable that more than one of these races would have an intersteller empire and that they would be in conflict. If our little solar system happens to be between two of these empires we might find ourselves in the midst of a war or the preparations for a possible war. It can be decidedly unpleasant to reside in a strategic location.
If the two sides are actively at war, they're likely to use our area of space without asking and might try to bully us into aiding their cause. If they aren't actively fighting but still hostile, they may try to recruit us as allies and offer us aid, a place in the galaxy, and protection from enemies. It's possible that we would end up with great prosperity, until the war comes and destroys us. A story to look for that deals with the problems of the alien allies is "The Liberation of Earth" by William Tenn; Earth finds itself first on one side and then the other over and over until it has no value to either side and barely is habitable by humans.
The problem we Earthfolk have in such a situation is that we aren't strong enough to be a neutral; we'd need to ally with one side or the other. If we're given a choice we might do better, but how can we really know if we chose the right side? If we had a choice between a race that looked like cuddly teddy bears and a race that looked like the creature in Alien we'd likely choose the bears, assuming that appearance gave a clue as to nature. Then we might find out that the bears are ruthless exploiters while the "Aliens" were really saintly. How do we know who to trust?
It is possible that our aliens are scared of our potential or our nature. If we were on the verge of achieving interstellar flight but had not yet become a peaceful people they might see us attempting conquest or getting other peoples involved in our wars. A race may simply be scared of any technological race, and seek to either conquer or destroy them in effort to neutralize the threat.
A relatively benevolent race would probably not destroy us, but would govern us in such a way that we're not a threat and would learn to be peaceable. A nasty paranoid race would likely destroy us; either by a genocide or perhaps just by destroying our civilization so that we could never rebuild it.
A particularly paternalist race might decide that the only way to neutralize the human threat without a genocide is to remove humans from the Earth and dispersing them throughout their empire in a sort of Earthling Diaspora. They might assume that it's our environment that makes us nasty, and if we're removed from that environment and the influences of our warlike governments we could become good citizens of galactic society.
In War of the Worlds and many other stories, the aliens send forces to engage us on the surface of the planet. With their incredible high tech weapons, they kick our behinds until something gives us a break so that we win or the aliens die.
This is the least effective way for the aliens to take over; a straight invasion. We humans fight lots and lots of wars and have lots and lots and lots of weapons. Unless the aliens have super miracle shields or something, they are going to take losses even if their weapons are vastly superior. Even if they "win", they're likely to have to deal with resistance fighters for a long time. Ground forces by themselves are a foolish way to attempt conquest.
The aliens can do quite a lot of damage without landing on the Earth; attack with beam weapons or projectile weapons from orbit or farther out. The Earthfolk would have a lot of trouble attacking the aliens or defending against the attack.
A lot depends on just how much of the Earth they want to keep; with beam weapons they could be very selective with their targeting, but large projectiles like rocks propelled by mass drivers would have a lot of collateral damage. Beam weapons would require very high technology and large amounts of energy, while projectiles provide a lot of bang for the buck and the "ammunition" is readily available in the solar system.
Such a bombardment would likely either be to soften up our defenses for an invasion or to impress us with the aliens awesome might and encourage us to capitulate without much of a fight. If the aliens goal is to eliminate us as a competitor, they might just bombard us until our civilization collapses which would leave the survivors as scavengers on the corpse of our past glory.
Biological weapons would be an effective way of reducing our population and threat potential. While it isn't likely that the alien engineered bugs would kill off everyone, they certainly could kill off most of us and leave the Earth as an easy conquest. Poisons would also work, but might kill off other parts of the biosphere that they would want to preserve.
To produce bioweapons that would be effective against humans but leave the aliens and the rest of the environment healthy would require test subjects. The aliens would need to capture and test their diseases on either humans or human tissue samples.
It would also be possible to make biological weapons that didn't kill people, but made them subservient. The aliens could spread a disease and then offer the antidote only if we submit to them. They could possibly produce something that would make us tame and obedient. The ultimate would probably be some sort of nanotech that would redo the whole of the planetary population into the loyal and obedient servants of extraterrestrial masters. One wonders just what the "Greys" are doing with their abductees, if there's any credence to be given to the reports of abductions; would it be research into such a pacification nanite?
If our aliens are actually from similar stock to humans separated from us in the past, or are able to disguise themselves effectively, they could be around us now. Why take over violently when you can take over control of the planet and/or prepare public opinion to accept you? They could be the "Secret Masters", having already taken over. :)
Alternately, the aliens might be taking people over, as in Heinlein's The Puppet Masters or duplicating them as in Invasion of the Body Snatchers. Such aliens might take over humans in positions of power or just take over in mass. They would be very hard to fight.
Symbiotic aliens might not be hostile or evil; it's possible that a race could be developed or engineered to live inside a host's body without harm to them. They might merge with the host like Trek's Trills; the resulting personality is a mixture of the host and symbiote's personalities. Or, the symbiote might live in the host with access to the host senses and possibly memories while contributing something to the host without controlling it. Such symbiotes might be created as memory stores, as in a third season Babylon 5 episode; they might be passed on to others as hosts die, retaining that life's memories.
Our aliens may not need to use violence to take us over. All they may need to do is offer us neat tech stuff; we join their trading block and end up controlled by their companies as they start to dominate our economy and buy up our companies. We could be "free" in name but actually be serving extraterrestrial corporations.
This might not be entirely bad for us; we'd be gaining a lot of technology that we otherwise might not get for centuries. Our trade with the aliens might cure a lot of humanities ills. We would gain a lot of knowledge about the universe. The drawbacks would be culture shock as Earthfolk deal with aliens and alien culture, and the possibility that we'd be second-class galactic citizens which might lead to resentment of superior acting aliens.
The aliens have taken over and installed their version of a government on our beloved planet. Most have grudgingly accepted the situation, but a few dedicated people are fighting the alien overlords. Our adventurer party is a cell of resistance fighters, or alternately the party is working for the (benevolent?) alien government routing out those nasty human terrorists.
Extraterrestrials have dispersed a large portion of the Earth's population throughout the Galaxy. Humans are a minority amongst a host of aliens. Our adventurers might be hired by aliens who think humans have some kind of special abilities suiting them to dangerous missions, or the aliens may just think that humans are the only race dumb enough and desperate enough to go on such fool's errands. In this setting, humans might be discriminated against by older and more advanced races; they might also be resented if they are successful. Look at the experience of various dispersed peoples in Earth's history for inspirations.
I don't think a war against truly advanced extraterrestrials would be terribly long, but an aggressive (and perhaps stupid) race that isn't much more advanced than us might engage us in a protracted conflict. Our adventurers could then be an Army or Marine unit that is stranded behind enemy lines, or has some sort of special function.
If our aliens wait until after we've developed more of a real space capability, we could then have a real space war complete with dueling space fighters. If our technology is advanced enough, the war might well extend outside our system to other systems. Our above noted adventure parties might then be fighting on an alien world.
In most alien invasion movies, the alien invaders somehow are defeated shortly before the end of the film; either they get sick or we come up with a super-dooper trick that gets around their super- dooper tech. However, we never find out what happens next with the exception of the television series that was a sort of sequel to the movie version of War of the Worlds; in that series we find out that everybody has forgotten about the invasion and everything is as if it never happened. Silly.
After the aliens are routed we are stuck with rebuilding. Many major cities are likely to have been destroyed along with a lot of our infrastructure. In a worst case scenario, humanity survives the war but our governments and economy collapse. Even after a "victory" it is still "the end of the world as we know it" and we might not feel fine, to paraphrase R.E.M..
This could be a good start for a "post holocaust" type campaign, perhaps drawing some inspiration from the hoards of movies along that line. There'd be some communities that are still at a 20th Century tech level, with some using alien tech to achieve higher tech levels. Other communities and areas would have reverted to more primitive technologies; the loss of population in the war would allow survival without a technological infrastructure. You might see feudal states, roaming nomads (they'd be on horseback; it's not likely we'd have "road warriors" in gasoline vehicles), anarchistic communities, and the whole hoard of historical and theoretical government types.
Our adventurers could be explorers from a city-state sent to see what has happened to the rest of the world. They might be primitive horse nomads, wandering the wilderness. Maybe they're traders, trying to set up trade routes between long separated communities. Or they could be scavengers in the ruined cities; living on old canned food and rats while digging up ancient goodies to repair or trade. Maybe they are an assortment of people brought together by bizarre circumstance. It could be fun.
Assume an invasion by aliens that don't have vastly superior technology. After crushing the invaders, we take their technology and conquer the galaxy. Oh, we might say that we're over dreams of conquest, but given the Earth's expanding population and the lack of much in the way of new "frontiers" to settle we at the very least would be looking for planets to colonize. Our governments would likely decide that in order to protect our colonies we must conquer the other space faring races with our superior technologies. See the story "The Road Less Travelled" by Harry Turtledove (in the collection Kaleidoscope, and perhaps elsewhere) for an example.
Our adventurers are now members of The Superior Race of the galaxy
instead of pathetic little primitives, if they're humans. They might
be on a new and dangerous colony, or exploring alien ruins, or working
on behalf of disadvantaged aliens. Alternately, our adventurers might
be aliens; see the above scenarios but substitute aliens for
humans.