In a lot of sci-fi, you see planets marred with battlegrounds, where species fight for dominance over x or y. This question kind of addresses those kinds of battlefields, but for a different reason. It makes sense to me that there would be battles over resource dominance on planets outside any civilization's jurisdiction.

In my world, there are a few major species that have interstellar colonization with jump gates, which if placed very far outside gravitational fields (i.e. at the outside of a solar system, weeks' journeys from inner planets by torchship) allow for instant travel to other star systems. They span much of the galaxy and sometimes find smaller civilizations that are more belligerent and would rather claim every planet for themselves than coexist with the other species (call it resource/land demand, speciesism, fascist WH40k Human Empire-style civilizations, or just having been taken over by power-hungry AI). This ends with interspecies war, although the dominant civilizations almost-always win.

What I would like to have happen

What I want to have happen is planetary-surface battlegrounds, like what's seen in Starship Troopers, Star Wars, Helldivers, etc. where there are actual soldiers on the ground that fight for territory/land/objectives on the ground. The kind of thing that can be advertised with "JOIN THE WAR EFFORT" to the rest of the major civilizations. For some reason, no matter whether or not there are resources or strategic points to be held on the planet, there should always be some reason to establish a battlefront there.

The problem I see with that

If a small belligerent civilization tries to attack one of the major ones, the worst they can do is launch a relativistic kinetic kill vehicle (RKKV) at a planet to sterilize it. Of course without a jump gate, that will take a very long time. So what I see happening is, instead of the major civilization sending in millions or even hundreds of millions of soldiers to beat the belligerents into submission, they just use their advanced telescopy tech to monitor the outer solar system for jump gates and set battleships next to every one they find. If they see an RKKV headed to a jump gate to get to another solar system to glass a planet, they just blow up the jump gate. Ultimately no actual threat is posed by minor civilizations to major ones that the major ones would actually even bother sending soldiers to the battleground.

The effect is that the belligerents have no way of actually hurting the major civilizations, and therefore even if they declare total war on every other species in the galaxy can be easily contained by just blocking the jump gates at the edge of the solar system with a couple of battleships, totally precluding the battleground I want to exist when such interspecies wars occur.


For what reason does an advanced, approximately-K2 galaxy-spanning civilization even bother deploying soldiers to the surface of planets of opposing, smaller civilizations if it is more practical, more cost-effective, and much safer for soldiers to just block the ways in and out of the belligerent star system? The goal is for there to be planetary battlegrounds that can be fought by soldiers on the ground rather than just a perpetual state of monitoring space near jump gates for RKKVs, because the former is interesting and the latter is boring, and also for Story Reasons the world needs to include battlegrounds.

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    $\begingroup$ It's your universe. Why not just make it impractical to block the ways in and out? I.E. "Jump gates are a hundred million kilometers in diameter and indestructible." $\endgroup$
    – g s
    Commented Jul 7 at 6:39
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    $\begingroup$ blockades don't solve the problem, there are numerous examples in world history. they just turn said place into a stewpot of resentment. Once you start a blockade there is no way ot end it without either making the problem worse or requiring you to put boots on the ground. $\endgroup$
    – John
    Commented Jul 7 at 14:52
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    $\begingroup$ I found this in the VTC queue. I'm not going to VTC, but it's worth pointing out that you're asking a popular question on this site: "how do mere mortals defeat my godlike character?" The answer is always the same, "they can't unless you introduce an exploitable weakness." Frankly, you're asking why Cuba doesn't (or never did) attack the United States? Answer: unless they had 100% backing from the USSR, it was suicide. So unless you expect the minor civs to be proxies in a giant cold war, you need to make your godlike civs mortal. Beware brainstorming, tolerance is waning for those Qs. $\endgroup$
    – JBH
    Commented Jul 8 at 6:13
  • $\begingroup$ This question is similar to: Can realistic planetary invasion have any meaningful strategy?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. $\endgroup$
    – Philipp
    Commented Jul 8 at 8:52
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    $\begingroup$ How tied are you to your space gate technology? Because you can easily justify a gate-based travel where only the source location needs a gate, and you "land" wherever you calculated yourself to land. That would open you up to people appearing anywhere (it would be safer to be on the outskirts, but all is fair in war), and you can't blockade in every possible direction. $\endgroup$
    – Flater
    Commented Jul 10 at 4:02

15 Answers 15


Because war isn't about destruction for its own sake, war is about doing, or sometimes simply showing you are capable of doing, enough damage to make the other side do what you want them to instead of taking the pain. Maybe that does mean plastering a thinly populated world so that other colonies roll over for you, and sometimes maybe you do have to kill a whole world but it's generally cheaper to visit a smaller, more focused, amount of violence on a very particular target. As Heinlein put it "There can be circumstances when it’s just as foolish to hit an enemy city with an H-bomb as it would be to spank a baby with an ax."

On the scale of stellar powers, even relatively small ones, it is virtually impossible to be sure of getting all the assets of a civilisation, space is too big, and if you do enough damage to piss them off without killing everything then they're coming back to get you one day. Cheaper in the long run to gain a true surrender enfold the remnants of their civilisation into your hegemony and shape them over the long term until they become you too.

Bringing war into the homes of a population is far more personally intimidating to them, and far more likely to sway them, than the impersonal though massive devastation of a planetary bombardment. The casualty numbers are so high as to be personally meaningless and relatively few survive to show their scars to the rest of the civilisation.

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    $\begingroup$ Don't annoy someone unless you're willing to follow through to the end; creepypasta.fandom.com/wiki/We_Know_You_Are_Out_There $\endgroup$
    – Richard
    Commented Jul 7 at 14:14
  • $\begingroup$ Most wars in human history have had some element of empire-building, and commonly the levying of taxes on captured lands. I guess it's possible to cleanse a planet of higher life in a way that preserves the fertility of the soils, but most leaders have preferred to keep some of the population alive despite the risks of insurrection etc. $\endgroup$
    – Stuart F
    Commented Jul 9 at 11:55
  • $\begingroup$ You don't even need to look to scifi to answer this question. Look at the USA's time in Afghanistan. This is a nation that has an enormous arsenal of nuclear weapons and overwhelming air power. They could have blockaded the country, or just bombed it flat. Why didn't they? Because it would have been entirely pointless and not served their goals to do so. They put troops on the ground, in small vehicles and on foot, because it was necessary to achieve their objectives in the region. $\endgroup$ Commented Jul 10 at 10:12
  • $\begingroup$ @anaximander Yes but then again no, Afghanistan is somewhat self sufficient in basic food, too mountainous for a total ground blockade anyway and honeycombed with natural and artificial underground hidey holes that make bombing everyone you want gone next to impossible without using nuclear weapons. Using nukes in a biosphere you have to inhabit is what you might call a bad plan and that's when you aren't doing it near neighbours with the inclination and ability to object in kind. $\endgroup$
    – Ash
    Commented Jul 11 at 5:44
  • $\begingroup$ @Ash A full blockade of Afghanistan would have been impractical, yes, but the US military could have managed blockades of fairly sizeable areas, akin to medieval siege tactics, so you can still at least partially draw parallels. And the argument against nuclear weapons works in the question scenario too - in a scifi setting you might not want to use overwhelming force from orbit because you actually want the planet to be habitable afterwards - immediately afterwards - and that requires a modicum of restraint and precision. Scalpel, not sledgehammer. $\endgroup$ Commented Jul 11 at 11:28
  • Bottled enemies could get ideas.
    They are afraid that an enemy held in their own system might research technologies to bypass the jump gate system. Why, until there were jump gates, people were confident that general relativity banned those, too. So they don't just want to contain the enemy, they don't just want to obliterate their main world, they want to shut down every last 'black projects' lab in the Oort cloud. That goes best by conquering them and having the Quisling government provide a full report.
  • The value of another ecosystem?
    It turns out that life isn't all that plentiful in the galaxy. There are lots and lots of prime terraforming candidates, and even more strip-mining candidates, but life is rather uncommon. To compensate, if there is life, an intelligence species is almost certain to develop sooner or later.
    The various terraformers and geneticists and ecologists do not want to miss the chance to investigate one more ecosystem. Perhaps it holds the moss which is 0.8% more effective on dry-and-cold terraforming candidates. Or the DNA-equivalent of the random rodent holds the clue to cure an inherited disease (by comparing their biochemistry with others and having the key insight). So there is a strong taboo against harming ecosystems, even enemy-held ones.
  • Their own values?
    Yes, they will genocide an aggressive species if they see no other option. But first, they will try to conquer and re-educate. The problem with that is the total war mobilization you envision -- wouldn't they switch tactics rather than introduce war bonds and the draft?

Here’s a few reasons I could think of:

Jump Gates are EXPENSIVE

I’m talking too expensive to destroy without a really good reason. Unless the state is facing total destruction, it won’t be worth it to destroy it.

RKKVs are also expensive

RKKVs require resources that are either

  1. rare and hard to extract, or
  2. required in massive quantities

So, these smaller states would likely lack the resources needed to create RKKVs in the first place. Think of them like nuclear weapons: dangerous, destructive, and really, really expensive.

There are easier ways to counter RKKVs

Shooting them down, bombs, destroying launch sites, etc.

Political and economic pressures

Destroying a jump gate would be very unpopular. It would effectively destroy trade routes, isolate colonies, and, importantly, tell the rest of the universe that this state cannot defend itself. After all, why else would they need to destroy a jump gate? (Whether this in-universe assessment is correct or not is up to you.)

Attrition sucks

The longer you wait for an RKKV, the longer the enemy can wear you down by other means. Blockades, smaller strikes, even cyberwarfare are all methods of attrition for the enemy.

It will all be over by Christmas!

The larger states think that the smaller ones lack the military to resist a planetary invasion. Therefore, it would obviously be much better to just invade them and get it over with. This would be a larger factor in a democratic state where ideas like this can quickly become popular enough to influence the government. However, this opinion could very well be false, leading to raging planetary battles just like you wanted.

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    $\begingroup$ A good way to make jump gates expensive is to declare they can't be built in place. They work by pairs and have to be built at the same location with their counterpart gate (the one ships use to jump from the mother system) because they require properly entangled neutrinos ("never mind, it's quantic"), and sent to the point of arrival by slowwer than light means. I.e. it takes at least centuries to establish a proper go and back communication channel with an outer system. $\endgroup$
    – armand
    Commented Jul 8 at 5:49
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    $\begingroup$ "It will all be over by Christmas!" <- I can't think of a single super power that has not made that mistake before. $\endgroup$
    – Nosajimiki
    Commented Jul 8 at 20:12
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    $\begingroup$ Something something "short victorious war." $\endgroup$ Commented Jul 9 at 11:04

The civilization isn't the people

Yes, the major civilizations can contain the minor ones. They can deny usage of the Jump-Gates, and they can blockade planets from orbit.

But then they are abandoning the people in those civilizations. Billions of souls, and all of their descendants, oppressed by their own governments, unable to live lives to their full potential.

Part of the goal of the major civilization is to give every sapient the right to travel, explore and reach their full potential. Those backward minor civilizations that use this freedom to invade and oppress other planets and even their own people are not acceptable.

K1/K0 wars are not a real war as far as K2 is concerned

Now, small scale wars and disputes are one thing. You disagree about the treatment of some people on another planet? You send soldiers over there to protect them.

Start engaging in weapons of mass destruction? Mass conquest or xenocide? That isn't allowed! Your civilization is sanctioned; reduced gate privileges, complete gate blockade, orbital blockade, and finally other civilizations (and volunteers from the major civilizations) are allowed to invade your planet so your local government can be replaced with one less hostile to the galactic community (and their own people).

The wars of the K0 to K1 civilizations is a kind of "game" to the K2 civilizations. They have rules about how you are allowed to fight. These rules permit massive battlefields and parts of planets turned to rubble, but don't allow slagging of planets or similar.

Even civilizations that disobey the rules are just constrained, and their planets get to be invaded by the rules. Aggressive warlike K0/K1 civilizations use these punishment wars as ways of expanding their own influence - they are permitted to land on the enemy planet and engage in brutal warfare, and the territory and resources they claim from these wars of punishment are partially kept as payment when the war completes.

Interplanetary transportation is limited

A well behaved K0/K1 civilization is permitted control over their own interplanetary craft and allowed to use the jump gates, possibly in a limited area.

If they are less well behaved, first they aren't permitted to use interplanetary craft outside of their internal borders; they have to be "carried" by better behaved civilization's craft. Eventually even internal interplanetary craft use is restricted, and then banned, as their violations grow.

A K0 civilization that refuses to follow the rules might have K1/K2 defended beanstalk on their planet, and they aren't allowed to fly ships into space; ships are just shot down. The area around the beanstalk is defended by advanced technology soldiers. Individuals are allowed to ride the beanstalk (after being searched), as are trade goods - and all interplanetary travel is riding on alien ships.

A K1 civilization that refuses to follow the rules gets bottled up in their system. A jump gate might be deployed and defended in the system; a transfer station where local "well behaved" ships are allowed to dock, and alien ships carry local goods and individuals through the jump gate, might exist if they aren't actively hostile. If the locals engage in war with the K2 jump gate operators, the absolute worst they can do is blow up the transfer station and try to take out the jump gate (isolating them), and even that is unlikely.

Better behaved K0/K1 civilizations are given the freedom of their system, and are allowed to transfer jump gates themselves. The cheap beanstalks and transfer stations still exist, but are less control points as economic resources.

Use of interplanetary craft to engage in warfare gets your entire civilization in trouble. Even Piracy gets you disowned by your civilization (but goes on regardless, unofficially; so long as the scale remains small).

More than one kind of war

So you can have two hostile civilizations flying through a system and not shooting at each other, landing troops on a disputed planet. On the planet they are engaged in a massive war; but orbital to ground weapons aren't used. Both are "well behaved" civilizations, so have their own interplanetary navies. Both engage in piracy against the other using deniable assets.

Observers and mercenaries also contribute to the war - observers are there to make sure the limits of the war are kept up, and mercenaries are there to fight for profit.

Another kind of war would be one where one of the parties is ill-behaved K0 civilization, and the other is well-behaved. The well-behaved party is landing troops in its permitted region and either defending a region, or attacking the other side. The ill-behaved side is denied access to space. Its (legal) imports are also limited, with naturally a thriving black and grey market of weapons technology being smuggled in.

A final, more violent case is a K1 ill-behaved civilization being punished. Here, the K2 and well-behaved K1 civilizations are attempting to punish it. One or more systems of the K1 civilization has a no-fly zone imposed on it, and the well-behaved K1 "helpers" and K2 enforcers are landing on planets and engaging in a war of correction.

Other parts of the K1 civilization may be permitted to continue to operate if they aren't actively attacking the jump gate and transfer stations.

In a more extreme case, the K1 civilization goes completely rogue. It manages to destroy multiple transfer stations and even occupy some jump gates. On the K2 side, the jump gates are isolated from the network, with stuff sent from them destroyed. K2 forces take system after system, including conquering the planets and "correcting" their people, with the goal of completely dismantling the K1 civilization and making it well behaved.

K1 "well behaved" allies fight along side the K2 civilization both in space and on the ground. The K1 civilizations get to exercise their interplanetary navies and troops, and some of the territory they conquer they get to keep (or get rewards from) later.

Categories used

I am using the Kardashev scale.

K0 is short hand for planet-bound civilization, like ours is. K0 is under K1, has less than a planet's worth of energy budget.

K1 is short hand for a civilization between K1 and K2. They have the energy budget of more than all of the energy that a single planet can capture, but less than all that a single star can produce (a dyson sphere or swarm).

K2 is what I'm labeling the major civilizations. They have multiples of star-output energy capabilities as a civilization, but less than a whole galaxy of captured stars to themselves.

Each of these has a power capability (in Watts) ratio of roughly 10^10 between them - a factor of 10 trillion. Civilizations a significant fraction of a K-scale below you are not a threat, except maybe to unguarded civilian individuals.

My prototypical K0 civilization is us - which is about K0.75, up to K0.9, a civilization bounded by overheating the planet from raw energy use. Pre-interplanetary, or at most minor colonies.

The K1 civilizations I'm imagining are K1.0 to 1.3 or so - that is 1 to 1000x the energy capture of a single planet. Interplanetary civilizations, with the vast majority of the civilization not on a single planet, probably bottled up by their lack of FTL.

K2 civilizations I'm imagining in the 2.0 to 2.5 range - 1 to 100,000 stars of energy fully captured, but still far from K3.


Have slow system to system ftl travel

Have it be possible for ships to do ftl travel much more slowly with system to system jumps. This means a primitive civilization can bypass jump gates and seek to attack a rival civilization. Once they get to the borders of them they can do constant low grade harassment.

Have very powerful planetary defenses.

Tech for planetary defense should be much more advanced. It should require an absolutely overwhelming and expensive force to defeat the full power of a planet, and so would generally be unrealistic.

What is realistic is having a fleet pacify a small part of the planet e.g. the poles and having troops land there. The troops can then destroy the ground based anti spaceship defenses and more of the planet can be claimed.


RHF and Ash did a nice job, though here's a few more points that could help you get a reason for boots on the ground.

Planetary missiles

If the natives are capable of sending RKKVs, or even just conventional missiles at the stargate/blockading fleet from planetary launchers like modern ICBM trucks and missile silos (though with a much more capable rocket), and the invaders have no realistic way of intercepting them in sufficient numbers or finding the launchers, such as by having cryogenic stealth missiles, then the fleet have a motivation to send troops onto the ground to find them and call in a nuke on them before the missile sneaks into a stargate and strikes a planet. This option may fit the Helldiver analogy particularly well, as it invites having a small squad on the ground to sneak around and search for their objective before calling the fleet to flatten them


As you have mentioned that civilizations with stargates build them far from their star, which considering the habitable zone of the solar system seems to imply that extremely capable torchships exist. This serves to reduce the cost of transporting troops to the target planet. Now that it is cheap to get boots on the ground,it could be tempting for the invaders to try to seize the planet with the infrastructure intact to serve their own economy if they already have to maintain a local presence.


The Tet offensive caused the USA to lose popular support at home as the citizens believed that they're losing, even though technically the US is the winning side in this battle. If a "defeat" at a distant land is enough to cause this, then what could destroying the stargate, abandoning whatever fleet and the crew that destroyed the gate in the first place in a distant system, and allowing the state that tried to commit what's basically genocide to get away with it do to popularity? The invaders could have a popular pressure for them to do something like a actual invasion.



The final segment of The Forever War by Joe Haldeman has the two species returning to far more primitive combat, with hand-to-hand weapons and bows and arrows. Dune similarly has everyone back to hand-to-hand combat. In both cases it's because shield technology exists which blocks all the more advanced weapons (or in the case of Dune, turns a more advanced weapon into mutual-destruction boom).

The Empire Strikes Back and Return of the Jedi had a related concept behind the attacks on Hoth and Endor. Planetary defenses are perfectly capable of stopping larger attacks: Hoth was well defended with cannon that could pick off any larger vessels with ease as they approached orbit; whilst shield generators on Endor protected the Death Star in orbit.

It's your world. You invent the technology which makes these limits happen. This is 100% plot-based so we can't help you with details, but this is where you want to be going.

Do think through your technology though. What are all the other consequences of this technology, as well as giving you the battlefields you want? And what are the consequences of those consequences? This is what gets you world-building that feels real.


Aside from the options other people have already mentioned, may I add:

Enemy has mixed feelings

Basically, the planets in question may have several factions. Perhaps the majority are in agreement that, yes, it's good to fight these random strangers for land and resources. But among the enemy, there are also factions that seek peace and may want to ally with the larger civilization. Diplomats and economists in the larger civilization will understand that they can definitely just snuff the enemy civilization with relative ease... but they also see sympathetic citizens on the other side of the aggression. Most people, from a moral perspective, probably won't like the idea of killing non-aggressors out of self-defense. But there's also the economic and social considerations. By providing a powerful image and clearing the way for sympathetic groups to establish governance over the former enemy territory, you leave a large, viable planet ready to be economically productive for you. It's happy to provide favourable trade deals or even join your empire, and better yet, the damage caused by the surface war will give your larger civilization a lovely opportunity to provide economic support, thereby creating dependence and asserting economic and political dominance for a long period of time.

So, for the cases where sympathetic groups are present, this lends the situation towards a fixed pattern:

  1. Allow the enemy to create aggression.
  2. Establish diplomatic ties with sympathetic parties; also, this is a great time to use the incoming conflict for propaganda internally.
  3. Supply sympathetic parties with troops, and allow the conflict to start on the surface.
  4. Naturally, you slowly crush the opposition. As your lines advance, you also make sure you supply the conquered territory with necessary food, shelter, and other support. This is to create an image in your new planet's eyes that your empire is both potent and benevolent. But at the same time, make sure there's some degree of destruction,
  5. Once the enemy is destroyed or otherwise neutralized (a surrender is great PR anyways), you help to establish sympathetic government(s) on the newly conquered world(s).
  6. You leverage your good relationship to get favourable trade deals, and supply as much of the reconstruction effort as possible, without rebuilding things too fast. Make sure they're dependent on your aid, and use that as a nice way to spoon feed them little bits of propaganda as well. Make yourself look good while profiting off the reconstruction effort.
  7. Once you've squeezed everything you can out of reconstruction and favourable trade deals, you begin making the correct propositions and pulling the right strings to get this conquered area to join you. At this same time, you also try to shift the public image of your empire away from "big, powerful, helpful organization" into "collection of different cultures and planets that are powerful in aggregate"; this is to make sure that when they join, they can feel like one among many rather than a foreign body which must lose its sense of distinction to properly assimilate.
  8. If all goes well, you have introduced one or more new planets to your empire, and have likely profited or broke even. You have a sympathetic government, good trade deals, and a generally accepting public.

You Aren't Really Fighting the K2 Civilizations:

Planets are next to worthless to the ACTUAL K2 civilization. Super-powerful AI run the true K2, and exists almost solely in orbital solar Dyson swarms and the like. The rest of the "civilizations" are glorified children who have access to only a fraction of the technologies and flit about like mice in the house of their Godlike neighbors and AI descendants.

This is not to say, however, that there isn't a supreme group amongst the children squabbling for planetary supremacy. The various Builders, the ancient races that gave rise to the AI, are the most familiar with the K2 civilization that they gave birth to. They've worked around the K2 for millennia, know how to game the system, and claimed planet after planet for their own. They may seem more advanced than their neighbors, and by comparison they are. But nothing like the K2.

So the Child races fight over planets because the K2 claims most asteroids, stars, uninhabited planetoids, and pretty much everything else other than the handful of habitable worlds kept pristine by the K2 AI's as a preserve for Child races.

The K2 let the Child races settle the planets and don't molest them. The children squabble, but they really need to learn to work these things out amongst themselves. On the rare occasions the K2 are challenged, their biggest concern is not hurting the children. Killing individuals of the species is inconsequential to the K2 AI, but like children stealing food off the table, sometimes it's better to let the kid eat the stolen scraps than to genocidally exterminate them.

The Child races are permitted to use the K2 gates & fly through their territory, but only as long as nothing they do threatens the K2. Blowing up a gate is a serious offense, or even a stray shot hitting a K2 facility. So it becomes problematic to use massive weapons of destruction. Damaging the biosphere of a planet is similarly frowned upon, so bombing a planet not only destroys the only resource available to the Child races, it provokes the ire of the K2 civilizations


Planets are the only game in town. It's incredibly hard to stop armies from landing on the planets. It's very difficult to simply exterminate your enemies from orbit. The gates are mostly held for common use by the K2 civilization. The Builder civilizations definitely have advantages, but not so overwhelming that they can't be fought.


Population control and morale

Why would any civilisation require more than one planet? Tge answer is easy. You need more living space. Unfortunately the growth doesn't stop there, so each civilisation based on expension will eventually reach limits that need to be enforced. There is a finite amount of space and resources, giving rise to many problems like hunger or not having an AR headset as big as your neighbour.

Taking a planet from another species would temporarily solve the space issue. In addition, you can slow down the increase of your species during. The best option of attack might in theory be high teck bombarding, massive swarms of drones hunting anything that moves or the like, but in practice it might be more economical to send the flesh and blood. You have a surplus of that resource. Many might not even be able to participate in the society, being just a drain of resources. That is why you send them.

It can benefit the whole empire as well, if primed for certain ideologies. Your propaganda makes them believe in the physical strength of the species, able to stare death in the face and triumph, expanding the glorious empire with victory. As mentioned in the movie Gladiator, the people of Rome loved each victory and expension of the empire, even though it was so far away it might not have happened at all fir them.

So controlling your population and the morale of your empire can be valud readons enough to send out the grunts. This is also beat for beat the Helldivers II story in between the lines.


The obvious question that jumps out at me is, if RKKVs are readily available even for the smaller civilization and not readily stoppable, then surely even more so for the advanced civilization, which means the real question should be - why doesn't the advanced civilization just glass the smaller civilization? That suggests that RKKVs (and WMDs in general) somehow need to be made impractical or undesirable.

Luckily, this doesn't seem hard to rationalize, because we can see the same processes on a smaller scale just by looking at our world. We have 3 very advanced civilizations with the capacity to annihilate any civilization they choose, a few smaller-scale but similarly advanced civilizations, and a number of civilizations with more limited capabilities. We've seen, over the past few decades, multiple conflicts between advanced civilizations and lesser powers, without the use of WMDs, including a couple that are ongoing right now (Russia-Ukraine, Israel-Gaza).

Once you see these parallels, you can make a lot of conclusions.

  • Often wars with smaller civilizations may actually be proxy wars with another great power (Russia-Ukraine, Vietnam). Great powers are likely to prefer such limited-scale conflicts to open confrontation. The larger civilization may be reluctant to use WMD to avoid provoking a reaction from the smaller civilization's sponsor. Equally, the smaller civilization won't use WMD even if they could, because that would justify retaliation, with the sponsor most likely staying on the sidelines.
  • Even if the other side lacks a powerful sponsor, using WMDs is bad PR. The advanced civilization likely prefers to present the war as "liberating" the smaller civilization from its current government (US-Iraq) or getting rid of the "bad guys" (US-Afghanistan, Israel-Gaza), both to justify it to its own citizens (especially relevant if it's a democracy) and because other great powers get jumpy once you start throwing WMDs around. That kind of rules out glassing their planets by definition.
  • The smaller civilization, even if it has WMD, is unlikely to use them because of the response. They'll prefer to try to survive and use guerrilla warfare to make the conflict too costly for the larger civilization in the hopes of regaining independence (US-Afghanistan). The only time they'd use them is if a) they're facing total defeat and b) annihilation is preferable to conquest. That, incidentally, provides a strong motivation for the larger civilization to be fairly restrained - they want to win without provoking the other side to launch WMDs. That being the case, they may not even attempt total conquest, they may prefer to occupy some strategic points and then negotiate.
  • $\begingroup$ Probably the simplest reason an advanced civilization would not use RKKVs to glass planets, is that while it is easy to glass a planet, it is rather hard, and expensive to un-glass one. By definition, planets with civilizations on them are hospitable, and thus extremely valuable real estate. $\endgroup$ Commented Jul 10 at 8:21
  • $\begingroup$ @GoingDurden That's a reason, certainly (and fallout is also a reason to avoid throwing nukes around), but it's far from the only or even the strongest reason. Even if a K2 civilization can't unglass a planet, they can drop down artificial habitats and collect whatever resources they want. In any case the point holds - WMD are a two-way street, and their non-use can be explained by observable real-world dynamics. $\endgroup$
    – user111403
    Commented Jul 10 at 8:49
  • $\begingroup$ the more cynical reason is that War is Business. Glassing a planet provides no profits to the Galactic Military Industry Complex. You need gun-totting Planetary Marines invade/liberate planets so that someone would buy plasma rifles and power armor. $\endgroup$ Commented Jul 10 at 9:02
  • $\begingroup$ @GoingDurden while a powerful arms industry is a relevant factor, I don't think it can be an answer on its own. They can't really make governments do impractical things just to sell impractical stuff (note how the sword and chainmail manufacturers are out of business), there has to be a plausible reason why land invasion is the way to go. Plus don't they really care what they sell as long as they get paid, it could just as well be developing the latest and best in unstoppable RKKVs and anti-spaceship missiles. $\endgroup$
    – user111403
    Commented Jul 10 at 11:30

What if the species didn't form a galaxy-wide of planet-wide government?

In other words, the planets would be, politically, like The Earth today?

We've got wars all by ourselves, and if a gate-travelling external party comes in with a small landing force, they can totally pit one Earth faction against another.

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    $\begingroup$ Cortez is a perfect example of this. His actual army was tiny compared to the Aztecs, but the small advantage he was able to offer to the Tlaxcala and Cempoala was enough to set off the powder keg. $\endgroup$
    – Nosajimiki
    Commented Jul 8 at 20:21

Easy: Costs and resources.

If building star gates, kill-strikes, space ships and other space-era stuff is ridiculously expensive, but soldiers and weapons for ground combat are relatively cheap, it is likely that the cheap option is going to be picked.

You can drive up the cost even more by adding some limitation to rebuilding. Maybe building a star gate is a manageable cost, but for some reason only a few spots around a solar system can be used for that, and if you blow up a gate at that point, it's not like all the rubble and fragments go somewhere. The cleanup mission can be many times more expensive than the construction.

Even human/alien life is relatively cheap in a galaxy-spanning civilization. We're talking a million trillion or so creatures. What's a million or two to die in a war? Population growth will replace them literally in less time than the war lasts.


Globalized Super Powers vs Insurgency

Interspecies trade is a big industry for all the major civilizations. They let millions of ships through thier gates every day because disallowing it would ruin economies and cause suffering on a scale bigger than you can imagine. A RKKV is no threat because it can be shot down very easily by an advanced civilization, but stopping gate traffic all together: trillions would stave, and hundreds of trillions of jobs would be affected. That is simply not an option.

Although they can stop war ships and ships equipped with anti-planetary weapons just fine, what they can not do is distinguish between a ship carrying thousands of tons of generic industrial manufacturing equipment, and thousands of tons of equipment that can be used to make battle droids, artillery, underground bunkers, etc. If your world already has pockets of immigrants and technology from 100 different worlds, that belligerent 101st species showing up wanting to set up shop is just part of the price of doing business.

The thing about advanced civilizations is that making a space laser that can level a city is easy, but when the enemy is already in your cities, using such weapons would be an even greater lose to your people than an unconditional surrender. This is especially true because a lot of your foreigners may be tourists, exchange students, visa workers, etc. So, even if you have the authority to nuke a million of your own people to win, nuking those 100 Telzallian missionaries could trigger a war with another K2 civilization.

So, to counter an insurgent army like this, you will need to fight surgically: and that means a ground war.


Make more Gates.

One thing you didn't address in your opening entry was how jump gates are constructed at the destination. If I am understanding correctly, these are wormhole gates that operate between two matching gates. Therefore, the very first thing I think is how did the gates get there? Who built them? How difficult are they to build? If it's doable, even if destroying the gates is a tactic, it may not be a long term solution. It depends which side we are talking about, and who can build the gates, but the point stands; make more Gates if they can.


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