Say portals randomly opened up on Earth that lead to an occupied alien planet, the denizens from one planet can survive on the other, and it's confirmed for both sides that the portals were not intentionally created as a means of invasion.

The personality of the aliens isn't too important. They are about as different as the most extreme human civilizations can be compared to the most mundane (which is to say, different, but familiar enough to understand basic concepts from the start).

What would cause the governments of both worlds to allow movement between the portals and NOT try to garrison the portals and invade the other world? Obviously not immediately, but how could this happen relatively quickly (within a generation, for instance)?

In case it helps, this is set in a medieval fantasy (few cannons, crossbows), but let's assume magic is unavailable (the exception being the portals, of course).


6 Answers 6


The same reason why most countries don't go to war with each other all the time. When we do wage war, it's often (not always, though) because it's necessary, or else if the balance of power is considerably different.

If the balance of power is roughly equal, then war is often devastatingly costly. Sure, you can potentially reap the benefits of newly concurred lands, but that can come at a cost that doesn't make it worth it.

And this doesn't even have to be a thought-out decision. We find co-operation in nature all the time. Altruism is an evolved trait, because it's often beneficial to co-operate than to fight. If you've read Richard Dawkins' The Selfish Gene, it has en entire chapter on this concept. There's a related BBC documentary as well, Nice Guys Finish First, and both are based on research by the political scientist Robert Axelrod. The chapter and documentary talks about the non-zero sum game Prisoner's Dilemma, which shows that in a society where repeated interactions are common, it's beneficial in the long term to co-operate than to fight to gain short term benefits.

So in your world, assuming both cultures are roughly the equivalent in intelligence and technology, it will be beneficial for both of them to not fight, and instead co-operate. Sure, there will be 'racist' in both parties, and there will be small-scale fights, murders, and the like, but both societies at large will figure out quickly that it's in each of their best interest to get along.

And it's almost certain that two such species will have things that the others haven't discovered, so they both almost certainly have much more to gain from each other than two human cultures who reunited after a long separation. So they will, almost certainly.


Details on how this might happen.

Let's suppose your portal opens not in secret, but pretty obviously in the public. Even in a medieval world news spreads fast and something like this will get the due attention in no time. Quickly the 'authorities' will get involved, and even thought the first travelers through the portal may have gotten killed (simply because we are afraid of the unknown), if they open long enough, curiosity will get the better of the... what are they called again? So we have good reason to believe one party or both would have sent deplomatic convoys, and assuming both have languages, it's not impossible to find a way to communicate.

The initial communication will probably be limited to 'government' authorities, but once they know enough to be less fearful of each other, people from each world can get access (maybe limited at first) to the other world. Quite soon the new becomes the normal, and after few generations nobody would bat an eyelash at the other.

Why would they want to co-operate?

Once you learn to not fear each other, it will be easy to figure out that they both have much to learn from each other. That's a big enough incentive to co-operate. Think about the mixing of cultures in our world during Medieval times. The Chinese traveled to the other parts of Asia and beyond, Arabs mixed with much of the rest of the world. They could have shut down all the naval routes and kept closed borders, but they didn't, because trade was lucrative. Everyone had something the others didn't, and it was in everyone's best interest to get along.

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    $\begingroup$ You make some good points on why they wouldn't go to war, but why would they allow movement between the portals? Even if they don't act against one another, the natural response would probably be to fortify your side of the portal. I didn't make the connection before, but Stargate comes to mind. Why would these communities be more open than similar examples in Stargate? $\endgroup$ Dec 5, 2019 at 21:44
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    $\begingroup$ I've answered that in the post as well. Once you learn to not fear each other, it will be easy to figure out that they both have much to learn from each other. That's a big enough incentive to co-operate. Think about the mixing of cultures in our world during Medieval times. The Chinese traveled to the other parts of Asia and beyond, Arabs mixed with much of the rest of the world. They could have shut down all the naval routes and kept closed borders, but they didn't, because trade was lucrative. Everyone had something the others didn't, and it was in everyone's best interest to get along. $\endgroup$
    – Sach
    Dec 5, 2019 at 21:49
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    $\begingroup$ @DanielZastoupil possibly because fortifying your territory outside the portal could be seen as an act of aggression and threaten any kind of peace $\endgroup$
    – BKlassen
    Dec 5, 2019 at 21:49
  • $\begingroup$ @BKlassen I meant fortifying your territory..from within your territory. Has a preemptive defense often been considered an act of aggression in our own history (and not as an excuse to wage war on the defender)? $\endgroup$ Dec 5, 2019 at 23:24
  • $\begingroup$ @DanielZastoupil I'm not an expert in history but consider the tension between North and South Korea, whenever there is increased military activity in North Korea tensions rise. This is the closest to your portal situation as I can think of, you can ask on history SE if you want a more definitive answer $\endgroup$
    – BKlassen
    Dec 5, 2019 at 23:38

How completely different is completely different?

If you make them really quite different then their desires and priorities can be far enough apart that there isn't any conflict. Medieval kingdoms tended to be all about land. Perhaps this other species only cares about shallow coastal waters and has no interest whatsoever in the dry bit in the middle. Or avians with a love of high mountains, maybe they need hot dry deserts to lay their eggs and too high a moisture content could be lethal.

The game with completely different is one of priorities. If they're far enough apart there will be no conflict simply because there are no conflicting requirements.


Why do you assume that they would go to war?

War is costly and often devastating, we don't wage wars simply because other countries exist. Your two worlds would benefit from mutual trade and the exchange of ideas. By allowing free movement people can enjoy the economic and social benefits of peace. The two worlds don't want to jeopardize that peace through aggression.

As to the issue raised in the comments regarding fortifications: consider the example of Canada and the United States. The two countries have benefited enormously from free movement (prior to 9/11 I could cross the border with just my birth certificate - no passport necessary). In sharp contrast, look at the situation at the Mexico/US border. The increasing fortifications have powerfully affected the relationship between those countries. Stronger border control is indicative of changing relations

Note: there may still be some form of customs or border control, but that doesn't necessarily mean"fortifications"

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    $\begingroup$ This answer heavily favours the portals opening in locations with minimal government oversight: that way, by the time they start talking about setting up borders, checkpoints, et cetera, there is already precedent to suggest peaceful cooperation. If the walls go up almost instantly, it encourages a very "us versus them" attitude $\endgroup$ Dec 6, 2019 at 16:20

Wars are not fought between entire species, they are fought between various social groups.

And wars are fought for good or bad reasons, that are usually a lot stronger than merely "Some other group of people exists so let's attack them".

History has examples of cooperation between groups belonging to different species.

For example, there are instances where dolphin groups and human groups cooperate to hunt fish.

In a region of Australia, a group of orcas cooperated with aborigines to hunt whales, and later cooperated with European whalers. In this example, it is at least slightly possible that both the orcas and the whales they preyed upon had intelligence equal to that of the humans involved.

And I find it easy to imagine that there could have been examples of cooperation between groups of early humans and groups of proboscideans (relatives of elephants) to dig water holes, for example.

And at the present time there could occasionally be small scale wars between chimpanzee bands, or between bands of Chimps or gorillas and bands of humans.

And when humans hunt elephants and/or take over their habitat, elephants sometimes strike back, with single elephants or herds attacking human villages.

And when there were several species of genus Homo co-existing relations between various small bands of the same or of different species could have been just a complex and varied as relations between modern nations can be.

So I can imagine wars between two different tribes of Cro-Magnons, wars between two different tibes of Neanderthals, Wars between a tribe of Neanderthals and a tribe of Cro-Magnons, wars between an alliance of Cro-Magnon tribes and a Neanderthal tribe, wars between an alliance of Neanderthal tribess and a Cro-Magnon tribe, wars between an alliance of Cro-Magnon tribes and an alliance of Neanderthal tribes, and wars between an alliance of Cro-Magnon and Neanderthal tribes and another alliance of Cro-Magnon and Neanderthal tribes, as well as various periods of peace between various such groups.

So you, the writer, the creator god of your fictional universe, should ask yourself:

"Why should two completely different species, newly adjacent to each other, go to war?"

Because no war mongers who may want the government of part or all of their species to go to war against some government of part or all of the other species are going to convince enough high ranking leaders and ordinary people that war is a good idea, unless they have some sort of reason, good or bad, accurate or false, to make it at least seem to a lot of their people that the war will be a good idea, whether it actually is a good idea or not.

So if you, the creator god of your fictional universe, want war or peace to exist in your story, or for peace to be broken by war, or for war to be ended by a peace treaty, you can arrange for that to happen.

And if you want the readers to desire war, or to want peace, or to change their opinions about war and peace, you can make them feel the way you want them to, if you are a sufficiently skilled writer.

And if you want to have a war between the government and society of part of one species and the government and society of part of another species to happen, you should take the time to give the sides motives for going to war that seem strong enough, at the time, to them.


Two different options, depending on the size of the wormholes.

If the wormholes are big enough for use to deploy planes/missiles through them, then they are also big enough for the aliens to launch similar weapons. Both sides would realize that it is a case of Mutually Assured Destruction, and you would end up with a Cold War-style "Well, I can always kill them, but it is guaranteed that they can also kill me."

On the other hand, if the wormholes are, say, Stargate-sized, then large-scale warfare would be completely impossible, as humans and aliens would be able to blast each other's armies the moment they come through.


Has no one here watched Counterpart? Counterpart

Basically a portal opened connecting 2 exact world. One difference in an event rippled through and caused most things to change ... Anyways the thing is that the 2 worlds are mostly same and how they interact with each other was truly fascinating.

Almost like what you guys discussed above like their govt interacting, how much knowledge was released, and how one side kept a backup plan to use in case someday things escalated [someone misused it]


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