I am working on a setting that involves multiple human civilizations, ending in a massive war in which one of them (an oppressive dictatorship) is defeated by the other major one (a democratic socialist republic and/or constitutional monarchy – haven’t determine which one).

My understanding is that intra-system wars are very difficult to sustain over large periods of time, mostly because there is no stealth in space – my understanding is that a war would tend to involve both sides sending out their entire fleets, the losing fleet being destroyed, and the losing side being forced to surrender.

Therefore, I plan on making the war being an intersystem war. My understanding is that that requires FTL for speed.

Are there ways around this? Or must FTL be assumed?

  • $\begingroup$ I am also working on a setting that involves a single civilization, ending in a massive war in which it defeats itself. My understanding is that intra-planetary wars are very difficult to sustain over large periods of time, mostly because there is no stealth on such a planet - my understanding is that a war would tend to involve humanity sending in their entire destruction power, ultimately losing. And there is no way around this, unless I assume FTL catapults or EMdrived crossbows. Otherwise no FTL without magic. $\endgroup$
    – user35577
    Apr 29, 2017 at 20:19
  • $\begingroup$ @qqjkztd There has been no all-out war between superpowers in the real world for precisely that reason. This war is somewhat like the Rebellion in Star Wars, except that the good faction is a nation that hopes to compensate for a numbers disadvantage with better tech, subtrefuge, resources from liberated territory, and disloyal enemies. $\endgroup$
    – Demi
    Apr 29, 2017 at 22:08
  • $\begingroup$ There's been no all-out war between superpowers because it would be too destructive (due to nuclear weapons) - not just to the military forces but to civilian populations primarily. $\endgroup$ Apr 30, 2017 at 1:19
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    $\begingroup$ See also How to avoid FTL as a plot device?. Your question is mostly a duplicate (“are there ways around this?”) but focuses on one specific plot. $\endgroup$
    – JDługosz
    Apr 30, 2017 at 6:12
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    $\begingroup$ Intrasystem wars basically boil down to who's at the bottom of the gravity well.. if I'm out in the far reaches I can keep lobbing stones at the planets close to the sun until one gets through.. in the outer reaches I've got plenty of space, plenty of rocks but limited energy and easy to get resources $\endgroup$
    – Chris J
    May 1, 2017 at 7:45

7 Answers 7


Nope. You don't need FTL.

Though you do need to move your thinking about warfare back a couple of centuries. And you need some seriously good environmental systems. Oh, and ridiculous engines. Oh, and some major production capacity.

Firstly: You need to stop thinking 'fleet' and start thinking 'armada'. Warfare in the age of tall ships is probably the closest analogue that we have here, when battles lasted weeks and voyages from country to country measured in months. Journeys to the enemy for you will take years, at best, so you need to take pretty much everything that you need to win the first battle, establish a beachhead and hold it until you can establish resupply. Given the timescales involved you'll need to have a trail of resupply missions following the initial hammerhead of your armada, and if the warfare is prolonged then each side will have vast amounts of resources doing nothing but shuttling back and forth in the space between stars.

Which brings me to resources and production. This is a major logistical nightmare, and your enemy has an insane home field advantage. Not only that, but you can't really call for help. You need to take as much production capacity as you possibly can with you. While certain things (people, high tech or sensitive equipment) must be sent from home en-masse, you'll basically want to be able to source basics like ammo, fuel and potables (air, food, water) when you get to the enemy star system.

Which brings me to recycling. This is where we need to get a bit soft-sciencey, or assume that your two sides really, really want to engage in this war. Basically the issue is that your ships will be leaking all the way to the enemy, regardless of how well you build them, and any losses on the way cannot be recouped, so either you need some super environmental systems, or you need to carry enough resources (air, mostly) with you that you can absorb any possible losses and not care.

Which brings me to engines. Unless you're planning on populating your warships with generations of some sort of engineered warrior culture (which is a really cool idea, but may be impractical) you need some serious propulsion. You're going to have to accelerate like hell until you get halfway, then flip and slow down. Doing this with an entire army, mobile refineries and mines, all your resources for the trip and initial occupation plus all the materials of war is going to take some serious, serious engines. When you take into account the tyranny of the rocket equation this only gets worse. You'll need something like Project Orion if you want to be hard science, but probably you'll need to go for something more soft-sci-fi.

Although taking all of this to its logical conclusion contested areas will have armadas of ships propelled by nukes filled with second generation warriors and military hardware being followed by endless streams of ships propelled by nukes filled with second generation merchants and material goods.

Or, to put it another way, space Constantinople.

EDIT: I Just noticed that the OP's question actually says intra system wars, not inter- system wars. This changes my answer a little, though only by relaxing the requirements somewhat as the travel times now go from decades to months or even weeks, and communication times now measure in minutes rather than years. This makes resupply easier and you no longer have to worry so much about environmental tolerances as you can probably set up regular resupply stations orbiting the star without too much expenditure. It also means that you don't have to have such badass engines. Indeed, if you really want to you could have solar sail propelled armadas.

Which really takes us back to the age of the tall ship!

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    $\begingroup$ If you're talking about journeys of years, you won't have accelerate-flip-decelerate (brachistochrone) trajectories. If your engines are powerful/fuel-efficient enough for those sorts of trajectories, most places in the Solar System are not that far away. projectrho.com/public_html/rocket/appmissiontable.php 0.1 g brachistochrone from Earth to Saturn - 52 days. A 1 g brachistochrone gets you anywhere within Neptune's orbit in 1 month. $\endgroup$ Apr 30, 2017 at 1:26
  • $\begingroup$ @cometaryorbit You know, I misread the OP's question as inter-system... I may need to make some edits... $\endgroup$
    – Joe Bloggs
    Apr 30, 2017 at 8:43
  • $\begingroup$ @JoeBloggs My question was phrases poorly. I did mean inter-system. $\endgroup$
    – Demi
    Apr 30, 2017 at 15:13

Don't forget fixed defenses. For example, build giant cannons on the moon. No atmosphere, limited gravity, and plentiful sunlight mean that a rail gun is quite possible. Spaceships incoming? Shoot them.

The main purpose of spaceships would be to destroy mobile attackers. E.g. someone aims an asteroid at your planet. You redirect it or destroy it. Or a combination of both. Redirect it and then break it up so that they can't direct it back. You may leave large objects in your orbit so that if they break up their asteroid, you can swat the resulting cloud with your asteroid. Or attack their ships before they reach your system.

Fixed defenses allow ships to drift back towards their own side when outnumbered. That makes things such that either side can defend easily, but it is difficult to attack. Thus a long war of attrition rather than a quick war of annihilation.

The problem with an interstellar war is that it's ridiculously difficult to attack. Without not just faster-than-light (FTL) but cheap FTL, it's not practical at all. Even Alpha Centauri is far enough away that soldiers will be too old to fight by conventional transport. But the energy costs are even worse. It takes nine times as much mass for propulsion as for the payload even for a ridiculously slow transit.

Even if you allow for some new form of super energy that produces more than $e = m c^2$, near light speed travel means that there are years after your decisive space battle before they reach the loser's system. The loser builds a new fleet by then. So your huge space battle doesn't actually decide anything. The defender can easily produce more ships, ammunition, and fuel. The attacker is too far from supply.

Also, why would you want to attack a society at another star? The level of energy needed just to get there is more than you could gain from the star. Worse, if the other side simply waits for you to get close, they can have a large energy advantage over you. Because the amount of energy needed to carry ammunition and maneuvering fuel is ridiculously high, the defending society has a huge advantage in any fight. The attacking society first has to secure resources in the target system just to fight moderately effectively.

If you really wanted to destroy a society at a different star, you'd send in agent provocateurs. They'd use the society's own resources against it. Because they wouldn't have resources of their own.

Soft science fiction generally handwaves these problems by making interstellar travel work more like medieval sea travel. Rather than requiring the entire production of an advanced society just to outfit one ship, it acts like rich individuals can buy them. FTL is something of a side effect, reducing transit times to something reasonable. But the real problem is more powerful than mass energy (MPtME?).


FTL isn't necessarily required for wars not to be limited to a single, decisive major battle.

Distributed nations / fleets

If the two powers at war aren't each a single planet, but each has assets spread all over the System (moon/asteroid settlements, O'Neill colonies, etc.), then the fleets won't all be in one place to have that one single large battle.

For example, if Earth and Mars are the capitals of the two nations, but both have colonies in the Asteroid Belt and the moons of Jupiter etc., if Mars sends a fleet to Earth, Earth's ships in the Jupiter system won't be able to get back in time to participate in the battle.

If the nations' economic power and populations are sufficiently spread out (not just one homeworld with minor colonies), most of the fleets might be distributed all over the place, so a single decisive battle would be very unlikely.

Expanding war

Even if the war ends in a decisive major battle, that doesn't mean it has to be brief. It could begin as a war between minor nations allied to the major powers, which then escalates as the major powers send in forces to help, then their forces clash directly, then the two go to all-out war.

Maybe both nations even have possessions on Earth. If space travel is relatively slow, you could have a war on Earth slowly expand into an interplanetary war as assets from progressively farther away get re-positioned and drawn in.


My understanding is that intra-system wars are very difficult to sustain over large periods of time, mostly because there is no stealth in space – my understanding is that a war would tend to involve both sides sending out their entire fleets, the losing fleet being destroyed, and the losing side being forced to surrender.

Consider the opposite possibility: Because neither side can effectively conceal their forces, and because of the significant amount of time in transit, the respective strengths of two forces are known long before they actually meet.

So if you know you're outnumbered and outgunned, you have the option to disengage rather than commit to a battle you'll likely lose. You can either break off your attack, or abandon your defense. This war might end up being a drawn-out, elaborate dance as the two sides posture and feint (with the occasional actual battle), until one can draw the other into a decisive battle that they cannot avoid (attacking their capital, for example).

But that's assuming both sides have a good picture of the enemy's capabilities. Even if there's no stealth in space, you can still disguise a warship as a civilian vessel, seize a space station with a guerrilla boarding action, or win a battle with weapons technology that your enemy is unaware of. Throw in that element of uncertainty, and you're likely to have battles where both sides think they have the advantage.

However you choose to approach it, the lack of traditional stealth doesn't necessarily imply a short-but-bloody war, and an intra-system war can have the narrative style you're looking for without requiring FTL.


Remember, too, that there are ways to target an enemy system without having ships onsite at all...

For example:

Strap a set of high-thrust engines to an asteroid. Tack a navigation/targeting AI to the front of it that's hardwired to the engines. Because no humans are on-board during flight, it can be engineered to accelerate to much higher speeds than an actual space craft could. Even with near-modern-day technology, we could probably have an asteroid going at a significant percentage of light speed by the time it reached a nearby star, if you can supply enough fuel to continue burning the entire flight.

At those speeds, an asteroid would be difficult to target by defense systems if they can see it at all. Remember space is really big and we can't even target slow-moving near-earth objects reliably. And if it should hit their planet or a major space station or whatever the target is, that target will be seriously damaged. One good hit of a large asteroid at 60% light speed would constitute an extinction level event for your target planet.

And at those speeds, hitting that inbound asteroid and making it split up may not stop the destruction; it might convert your high-velocity sniper round into a shotgun blast. No less deadly to those on the planet's surface, either way.


It seems more likely to me that "war" between groups at immense distance would be more like the trench war impasse of WW1. Each side would accelerate asteroids and send robots across the void at each other, and try to pick off or divert incoming projectiles & autonomous drones. This kind of war can go on for a long time, until trouble at home means the defenses are not maintained and a dinosaur killer gets through.

Even with current drone tech, a bunch of guys on a ship seem like a tempting target. I am not sure what use the space Spanish Armada would ever be.


If you want to avoid FTL, I would suggest keeping it intra-system and having previously built asteroid bases about halfway that produce ships and equipment while also providing protection. Needs a bit of handwavium, but less than for travelling between star systems at sublight speeds.


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