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In this scenario there are two spacefaring empires, each with a handful of solar systems under their control. The two empires have been adversaries for millennia.

In each solar system there are habitable worlds. Any indigenous population on these worlds was forcibly conscripted into the empire that found them first and operate as a vassal state. Unoccupied worlds were populated.

The technology in this scenario allows for planet-to-planet travel in a matter of hours. Imagine 1 hour from Earth to Mars, or 10 hours from Earth to Uranus for scale, something akin to air travel for us now.

System to system travel, while faster than light, still takes centuries. Neighboring systems might be 200 years, while the frontier systems may take 800 years or more to reach from the core worlds. Most of the beings in this universe have a life span similar to humans - assuming technological advancement in health care and gene-therapy, maybe 200 years. Cryogenic suspension is possible but difficult.

The question is: What does this war look like?

Sending a fleet to assault a system would take hundreds of years and several generations, and would be seen coming decades before they arrive. Also, those ships would be unavailable to counter some other offensive launched against their civilization.

I have a few options on the table already:

  • Attaching drives to meteors or other large space rocks and launching towards the expected location of the system. These could travel faster than the ships, maybe decades instead of centuries. I assume these species could to the math for where to aim them.

  • Ships full of robots, again these could go faster than 'squishy' ships. They would be hard pressed to accommodate changing commands though.

  • Ships with no crew but full of bio-weapons.

Note: For the sake of the story, let's assume communication is near-instant across systems.

Thanks!

Update:

Just to provide some clarification, each side of this war has a deep-seated, almost religious belief that their empire must be the one to control the stars. There's nothing political about the war, it simply must happen, and must continue until one side wins.

Second, neither side wants to rule over a galaxy of glass balls in space. They know that habitable planets and populations are valuable. Each side wants to conquer the systems while still having something to rule. Since all but the original system for each empire was forcibly conquered in the first place, a change of allegiance is realistic. (Perhaps several systems have been passed back and forth already)

They would be willing to 'nuke' a planet or two if necessary, but for the most part the goal is to take control of the planets under the other empire's control.

The 'vassal state' planets are effectively doing their own thing in terms of society, but they are directed by the overlords to help the war effort - for example 'attack planet x' or 'build shipyards'.

Update 2

First of all, thanks everyone for the feedback and ideas. It's a lot to wade through and there a lot of ways this could impact the universe I'm thinking of, I'm going to need to unpack it all in my brain.

Second, I need to apologize because I muddied the question by referencing FTL. My intention was to ask what a war would look like without planet to planet travel times being short, like in Star Trek or Star Wars. Going back to my old math / notes, the speed I'm looking at for ships is about 10% of Light speed, or around 1,000,000km/h.

At this speed, Earth to Mars would be less than an hour, Earth to Uranus about 25 hours, and Earth to Alpha Centauri would be 42 years, Earth to Wolf1061 would be 140 years, etc... Not accounting for acceleration issues, if there are any with this level of technology. I want it to be somewhat science-based but a little handwavium is OK to keep the story entertaining (it will use real star systems and their real distances for example, but somehow maybe the aliens all have a universal translator).

Third I think technology of the two main factions would basically have plateaued. The vassal planets might not all have an even level of technology, but overall there aren't any game changing discoveries that change the balance of power on a regular basis any more.

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    $\begingroup$ I do not see why civilizations would continue a war when just attacking an enemy takes longer to happen than a single lifetime. If you are a leader of such a race, there is no satisfaction (or political capital) from launching such an attack (which will cost resources immediately and hence both political and financial capital for no gain) when no result will appear in your lifetime. Also robot fleets designed to attack planets and acting autonomously - sound more like creating a new problem than solving an existing one. $\endgroup$ – StephenG Oct 6 '20 at 2:08
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    $\begingroup$ Centuries of travel time between worlds precludes the idea of empires. There's no communication, exchange of culture, commerce, or any practical remittance of taxes to maintain a unified empire or any other type of unified polity. Worlds would be pretty much doing their own thing by themselves and would rapidly declare independence. $\endgroup$ – GrumpyYoungMan Oct 6 '20 at 3:35
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    $\begingroup$ How would you know that there's an enemy out there, if the messages took centuries, or at least decades to reach home? Also, war speeds up technological advancement tremendously, so probably the second wave of generation ships/war fleets will be overtaken by the fourth. $\endgroup$ – Erik Oct 6 '20 at 7:39
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    $\begingroup$ I applaud OP's recognition of the fragility of "squishy" lifeforms, but remind him that tolerable acceleration needs to be considered for interplanetary travel which suggests that Earth to Mars would probably be days rather than hours. @GrumpyYoungMan hence also Krugman's 1978 paper on interstellar trade princeton.edu/~pkrugman/interstellar.pdf $\endgroup$ – Mark Morgan Lloyd Oct 6 '20 at 10:11
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    $\begingroup$ "System to system travel, while faster than light, still takes centuries. Neighboring systems might be 200 years [apart]" Are you assuming most systems lack habitable planets? Or much sparser stars? Because even at c travel speed, we can reach our neighboring system (Alpha Centauri system) in under 4.5 years (under 4.25 to reach the outlying star of the system, Proxima Centauri). 200 years at even low FTL speeds means an absolute ton of star systems; Earth has roughly a quarter million stars within 250 light years. $\endgroup$ – ShadowRanger Oct 6 '20 at 13:46

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Your war needs an exceptionally strong reason to fight

The war needs to have some fundamental underlying driver beyond almost any existing war in human history. No resource shortage, no class divide, no empire expansion desire, no desire to self govern, no desire to liberate captured land, nor any other war justification in history, is going to survive 8 generations of travel.

I will fight to free myself or my children from suffering. I may fight to avenge my parents. Grandparents if exceptionally motivated. 8 generations back? I just can't get motivated enough to give (yes - give, I'm not going home even when we win) for the Bolivian War of Independence or the Battle of El Toro, nor any other conflict that affected people 8 generations back. I don't believe propaganda could stretch this motivation out for 200 years; brainwashing from birth might work for a few, but you want your military to be able to think on the move - and that teaching them to think can break centuries old propaganda.

Thoughts on what causes that may survive 8 generations:

  • Religious indoctrination / crusade. You brainwash the children on the generation ship to wipe out evil non believers.
  • Some pure indisputable evil force. Literally Satan sort of thing.
  • Some out of control tech which can't be negotiated with (eg an out of control AI that wiped out its people).
  • Some enemy who will attack you for a fundamental need. Eg Wraith from Stargate, or some other smart monster.

The standard "We better attack now because they're about to attack us" line used to start most wars of aggression wont stand up to 8 generations of scrutiny; it's only been a few decades since these lines:

... and already we can dismiss these as fiction / propaganda. Few people today believe Poland attack Germany first, or that Iraq had a serious large stockpile of WMDs.

What would the battle look like?

Your generation ships are going to be the centerpeice of your fleet (they're big, important, but vulnerable). Escorting these will be smaller, faster, remote-controlled fighting ships which do the bulk of the work. Basically a carrier battle group. Carrier in the centre, smaller ships around it with specialised weapons-platforms for each potential threat (Air / sea, submarine). You'd have a similar variation:

  • Drone ship - that drops small fighting drones.
  • Big gun-boat style ships for close in annihilation or wiping out incoming projectiles in a hail of flak.
  • Missile frigates for precision distance attacks.
  • Carrier which can launch lots of robotic small fighting craft.
  • Artillery Ship with a big central gun for when massive firepower is required.
  • etc.

I'd have an automated defence drones that hides itself with close-to-ambient-as-possible temperature and passively scanning for threats, and powers up and shoots anything that doesn't pass an IFF challenge. Deploy these liberally around your systems for defence.

Then launch your fleets. As your fleet travels, they should distribute these defence drones into no-mans-land between your systems, over as wide an area as possible. Basically trying to mine the good transfer orbits between the star systems. These will need to be redone every few decades, (or the drone mines need to power up and move every year or so) as their orbits will drift.

While in transit, your middle generations have nothing to do other than raise their children, so have them upgrade the robotic fighting ships as new tech is discovered. Your only a few light years from your home base you can get data bursts of new tech. A modular design for ships, a dry dock in the generation ship, and onboard 3d printers should help keep the robotic fighting ships mostly up to date in transit. (No they wont be as good as freshly made ships, but it's better than nothing).

As you approach an enemy system; Decelerate your generation ships before the bulk of your fleet. The remote controlled ships go in and do the battle, the generation ships stay back, using remote control to keep themselves as far away from the fighting as possible, but close enough that they can direct the battle without serious transit time issues.

Once the defences are destroyed, then bring your generation ship in to land and use as an occupying force.

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    $\begingroup$ "Once the defences are destroyed". By a fleet that's a millenium out of date? $\endgroup$ – Mad Physicist Oct 6 '20 at 13:36
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    $\begingroup$ @MadPhysicist - communication with home base hasnt stopped (they're only a few light years from home, they can get data bursts), so as new tech is discovered in many cases it can be integrated with the ships in transit. If you have 200 years to kill, your on a massive ship with thousands of people and everything they need to live and maintain a fleet, and you know of some upgrades, you'll be able to do many of them. You probably can't majorly upgrade the gen ships engines or power gen, but you can dry dock the robotic vessels that actually do the fighting and do many upgrades on them. $\endgroup$ – Ash Oct 7 '20 at 1:28
  • $\begingroup$ +1 and agree with most, but be aware that there were instances in Ireland last century where the motivation for attacks were actions by the British in the 1600's. Some people really can't let go of the distant past, no matter how incomprehensible that attitude is to either of us. $\endgroup$ – KerrAvon2055 Oct 7 '20 at 22:08
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    $\begingroup$ @KerrAvon2055 I don't think that's really an accurate characterisation of what went on in Ireland in the 20th century whether you are talking of the War of Independence or the Troubles. You could trace back historically but both were a consequence of the continuing presence and actions of the British in the then present. Whether you agree with them or not is not the point- the point is the conflict was down to the British still being there and what they were getting up to, not the plantations of the 1600s. And for sure there were very legitimate grievances whatever of the response. $\endgroup$ – Ivan McA Oct 8 '20 at 9:41
  • $\begingroup$ An American analogy would be to say the grievances of the the modern American civil rights movement is people motivated by the actions of those who took part in the Atlantic slave trade which also started in the 1600s. That was a historical wrong... but there were (and are) plenty of contemporaneous problems that are really more the immediate issue. South Africa was also first colonised in the 1600s. Were the opponents of apartheid there people who "really can't let go of the distant past"? $\endgroup$ – Ivan McA Oct 8 '20 at 9:46
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Since you seems to want two fairly evenly-matched sides, there are three broad possibilities for what a conflict will look like:

  1. A Korea-style defensive stalemate. The cost of attacking prepared defenses is so much greater than the cost of building and maintaining those defenses that a successful attack is essentially unaffordable. Wait for the other side to collapse due to some other cause.

  2. A Cold War-style offensive stalemate. Mutually-assured destruction using unstoppable planet-cracking FTL warheads. Wait for the other side to collapse due to some other cause.

  3. A World War I-style vast war of attrition that will drain (and eventually overthrow) both sides. The 'overthrow' result is not trivial, so any smart Royals in your empires might steer toward one of the other stalemate options in order to keep their head.

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    $\begingroup$ Backing up the first possibility is technological advancement — by the time a fleet arrives, it'll be centuries behind the opponents and likely easily crushed. $\endgroup$ – parasoup Oct 6 '20 at 3:21
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    $\begingroup$ @parasoup To me the setup (long travel, next to no cryosleep) basically demands big generation ships, which would include their own production facilities. So a lot, if not all of the technological progress could be reproduced en route. In fact it might be a cost saving measure if one can convert some of the mass of the long-term systems no longer needed (e.g. food production) into weapons on final approach, instead of having to carry both. $\endgroup$ – mlk Oct 6 '20 at 11:11
  • $\begingroup$ Technology change or clever ship design might indeed shift two evenly matched sides among (1) (2) or (3). But notice how it rarely improves the final outcome. $\endgroup$ – user535733 Oct 6 '20 at 13:59
  • $\begingroup$ Faster than light planet-crackers would also require some kind of FTL radar, otherwise they might make a reaally unstable stalemate. $\endgroup$ – ilkkachu Oct 6 '20 at 20:38
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    $\begingroup$ @ilkkachu, space is <s>an ocean</s> big. Just hide your <s>missile subs</s> planet-cracker bases out in the middle of nowhere, and you can be assured that a retaliatory strike will be launched. $\endgroup$ – Mark Oct 6 '20 at 22:24
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The only option I see with an actual chance of winning would be a "colony-hopping"-approach. The reason for this is very simple:

A travel time of 200 years means that the attacking side simply has no chance. Not only because they will always be 200 years behind in technology, but because of the supply line situation combined with the generally stronger position of the defender, who has all the time in the world to prepare.

Therefore the only rational approach would be to gradually establish colonies/outposts into the direction of the enemy, thereby constantly closing the distance with the goal of establishing an "assault-colony" near the enemy empire, that is capable of building up all the necessary infrastructure, manpower and ships to launch the real invasion.

This would be obviously be a an endeavour of enormous proportions, taking place probably over millenia and requiring a zealous intergenerational devotion to it, which is given by your parameters.

What makes this approach especially interesting is the very likely prospect of both empires doing it simultanously, which would lead to a massive clash somewhere in the middle between assault-colonies that only exist to invade the enemy.

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    $\begingroup$ This is going to be a lot harder than just "Build a chain of colonies in the middle of two systems". Over milinea the colonies built between two star systems will drift (2 adjacent star systems are in different orbits around the galactic centre). The drift in relative orbital speed is slow but your colonies are going to be slowly drifting out of position. A line of colonies every light month between 2 stars will not be useful stepping stones after a few thousand years. (After a billion years they'll be scattered around the entire galaxy in a ring) $\endgroup$ – Ash Oct 6 '20 at 9:28
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    $\begingroup$ Is the drift severe enough for that to be relevant considering a timeframe of only a few millenia? If so, it would probably render the whole question moot anyway. $\endgroup$ – AuronTLG Oct 6 '20 at 10:05
  • $\begingroup$ @Ash Why would they drift? Wouldn't they have the same momentum as the starting system? And even if not, couldn't their course be corrected? $\endgroup$ – Jann Poppinga Oct 6 '20 at 11:15
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    $\begingroup$ @JannPoppinga. Something between solar systems is not free from gravity, it's in a galactic orbit. if they have the same initial speed but different distances from galactic central point they'll have different orbital parameters. $\endgroup$ – Ash Oct 6 '20 at 11:45
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    $\begingroup$ Or put another way. If I put something half way between Earth and Mars at the average velocity of both, I'll have another body in orbit around the sun. A few hours after creation it wont be in the middle of the two bodies anymore. And 9 months later itll be be on the wrong side of the sun. $\endgroup$ – Ash Oct 6 '20 at 11:52
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Special relativity forces FTL travel to be instantaneous...

... at least from the frame of reference of the people travelling.

Yes, I'm aware that faster than lightspeed travel isn't accounted for by special relativity, but you're asking for a science-based answer on top of the existence of FTL travel.

Travelling at exactly lightspeed would effectively render time to stand still (for the traveller). It stands to reason that going even faster, if that is assumed to be possible, is not going to "undilate" time.

However, you can still have this journey take centuries from the frame of reference of the people inside the solar systems. But this does change the basis of your question. It becomes a sociological issue, rather than a technical one.

Using a real world example, where you can assume lightspeed to be the maximum achievable speed, the Sun and Earth are 8 minutes apart when moving at the fastest possible speed.
Assuming we had the means to immediately travel at lightspeed, let's say something is happening on the surface of the Sun that requires our immediate attention. For the sake of example, let's say the Sun is going to burn out unless we plant this magic rock on its surface.

If we wish to intercept the sun burning out, then we MUST leave on that mission at least 8 minutes before the sun will actually burn out. That's simply because we must account for travel time. If we start travelling with only 2 minutes left on the deadline, then there's no point to even going on the mission.

Going back to your example, if your travel takes 200 years, that means that you must leave on your mission at least 200 years before it's too late to do so.

You clearly have some maximum travel speed in mind. It's faster than light, but there's still a maximum. Based on that maximum, there will be a certain travel time, which in turn will be relevant for every decision on whether to send people on a given mission or not.


Technological advancements over the centuries

So, we're sending our finest troops into combat. It will take them 200 years to get there, but it will feel like a fraction of a second to them.

Essentially, we are sending troops with weapons dating from 40 years before the US civil war to fight modern day Navy SEALs. I actually cheated in your favor, the first picture is from during the Civil War, 40 years more recently than the actual guys you'd be sending. It's not hard to see how they are going to be at a tremendous disadvantage.

Realistically, for a 200 year travel window, you would need to be 200 years ahead of your opponent in order for your troops to arrive with a fair chance of winning an engagement.

But it gets even worse. You have stated that there is near-instantaneous communication between these systems. That means that the people in the target solar system can be warned 200 years ahead of the arrival of the troops. Not only can you forget about shock and awe tactics altogether, your enemy has several generations to prepare for your arrival.

Worse still, your technological advantage can be undone if your technological advancements can be transmitted to the enemy forces - who will be able to develop that technology much faster now they have your plans.

We're sending civil war troops to engage Navy SEALs, and the SEALs know you're coming. And they've had literal generations to prepare for your arrival.

Good luck with that.


Zealotry as the "solution"

Just to provide some clarification, each side of this war has a deep-seeded, almost religious belief that their empire must be the one to control the stars. There's nothing political about the war, it simply must happen, and must continue until one side wins.

However, you seem to be describing a situation that is going to be rife with zealotry. That works in your favor.

Rather than finding logical and scientific reasons for why this engagement is going to be an uphill battle to say the least, zealots following dogmatic ideals tend to gloss over those kinds of considerations.

I would expect that the only people who'd still engage in this ware are zealots who are convinced that:

  • This war will never end, the enemy will remain our enemy in perpetuity.
  • "We" are invariably superior to our inferior enemies
  • "God" will make us victorious above all else
  • All of the above will trump any possible technological advancement the enemy may have.

Essentially, you need militaristic egocentrical space zealots.

It's very possible to create this kind of civilization. Whether the citizens themselves are such zealots, or they simply churn out a clone/cyborg army to do the work for them (with the actual citizens knowing "better"), it can be done.

But this kind of civilization, and the sociological issues that readers are going to pick up on (essentially space nazis) are going to very much attract the spotlight of the story. It's going to be hard to write a story with a satisfying arc where these problems do not get addressed.
Again, not saying it can't be done, I'm just warning you about how much of a magnet for plot attention this civilization is going to be.

It actually reminds me a lot of the Warhammer 40K kinds of civilizations. These are built with strong tones of militaristic religion ("inquisitor", "heretic", ...), and built on an age-long conflict between races that do not inherently change at their very core. The technology may change, but the attitudes towards each other do not.

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  • $\begingroup$ I believe there are more motivations for millennia-long warfare than just "space nazis". It could be biological imperatives - e.g. "eww, space spiders" vs. "eww, space frogs". It could be incompatible philosophies - pacifists vs. honorbound warriors (reinforced by evolution, e.g. one homeworld rewarded inter-pack cooperation and the other rewarded inter-pack conflict). $\endgroup$ – toolforger Oct 7 '20 at 7:18
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There's no science-based reason they would have to be seen

If your ships are traveling FTL, then there's very little chance of them being spotted unless you, the author, invent a reason. They're out-racing their own light, so the ships would arrive before the enemy star system would see the light of their initial launch. As long as you have fairly good sensors and put a minimum size on FTL vessels/transmission equipment their arrival will be a surprise. (such that, for example, your civilizations can track anything bigger than a car within their systems and FTL ships must be at least Aircraft Carrier size, or that FTL comms are big and obvious so a guy in a house can't just call the target planet on his cell phone when the fleet leaves.) Even if they need to exit FTL at the system edge and use their super-fast "normal" drives for the final approach that's only a couple day's warning, which isn't insurmountable.

As to the potential centuries of tech improvement by the waiting enemy, that's also somewhat overcome by history. The pace of technological advances in the past few centuries is unheard of in human history. It's been increasing, but historically relatively small incremental change over centuries or even millennia has been the norm. All if would take is a throwaway line (or a basic assumption left unsaid) that the "technological singularity" everyone is currently talking about was actually a dead-end. No "Oneness with the machine", turns out that after FTL and quantum computers damn near everything else is impossible! Then it becomes the difference between the Brown Bess (workhorse musket of England designed 1772 and used until 1836) and the model 1858 Springfield (common in the American Civil War and technically a superior weapon by 85 years). The fighting happens the EXACT SAME WAY with both weapons. Then the arm's race becomes novel ways of using old devices or desperate gambits where fleets are launched with some Great New Wonderweapon with the knowledge that it took 2 centuries to produce and previous wonderweapons have taken 2 centuries to overcome. So if your travel time is 180 years you might just overwhelm the enemy! Not a system a ration civilization would use to launch millions into the void, but religious zealots? They're rushing to the recruiting stations!

If both these things are true then you'll end up with two VERY belligerent, evenly matched galactic superpowers duking it out in the Grand Old Space Oprea Style! I can imagine that the troops actually involved in the fighting think of themselves as an elite race apart (fitting, for the religious zeal your civilizations have). Travel for them feels instantaneous, so they go from battle to battle aloof from the material universe (and largely not knowing about it, as they lose centuries at a time!) making war as their High Priests will in the name of their God.

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Cyber Warfare

If travel takes centuries but communication is instantaneous, there would be a very strong incentive for informational warfare. Your empires would try to convince locals on the enemy planets to fight for them, launch virtual attacks, broadcast plans for building nukes in your garage, spread disruptive rumours...

Defense is easy

To actually capture the thus weakened planets, you'd need a massive invasion force. Defense would probably be easier than offense, especially if you want to take the enemy planets intact. Any attacker would be seen for many decades before arriving, which allows the defender to prepare an appropriate reception - possibly including asteroids slung in the way of the attacker, which would impact at more than 10% of light speed due just because of the travel velocity of the attacking fleet!

Tech Rush/Eco Rush

One way to counter this advantage would be an even greater economical or technological advantage. You would want to play the long game and try to improve your capabilities at a faster pace than your enemy, until you felt confident in your ability to overwhelm any defensive system they could deploy in the 100 years of travel time.

Mind Games

You could improve your economical advantage by making the enemy waste resources countering "attacks": if you launch asteroids painted like death stars at them and tell everyone how powerful they are, they might spend a considerable amount of resources trying to intercept them, giving you the opportunity to save up for an actual attack. Bonus points if once in a while you mix in an actual generational attack craft full of smaller combat vessels, which would try pierce the front lines and damage or destroy major economical installations (shipyards, factories and energy producers, but also population centers). They could also play dead or hide inside an asteroid ring, to ambush transport ships passing by.

Asymmetrical Warfare

If you are able to penetrate the defensive line even with minor assets, guerilla warfare becomes hugely attractive. Orbital mechanics make pursuit much more expensive than evasion, and there are bound to be soft, valuable targets even a lightly armed vessel could destroy. Having to defend every factory complex and shipyard against possible sneak attacks also places a huge strain on their resources.

If you dislike asymmetric warfare, there is another option. The closer you are to your enemy, the less time he has to prepare for attacks, and the better you can judge the state of his defenses. Therefore, reduce this effective distance:

War Factory

You build a gigantic habitat and production center, continually supplied by asteroids shot after you from your home system. It processes these into weapon systems, ships and potentially even more interstellar factories. Because of its size, it gives a defensive and logistical advantage close to that of a moon base (perhaps it even is a hollowed out moon) and travels slowly enough to easily evade or deflect incoming asteroids. Since it is upgrading and extending itself all the time, traveling at only 0.02 c is actually not a problem: it only allows you to receive more resources and build more offensive and defensive assets. Once you are in range, you can fight a war without all the logistical nightmares of combat vessels 200 years away from their base, and as long as you resupply your station with raw asteroids (which are relatively easy to launch with gigantic magnetic cannons or accelerated on laser sails) you can continually resupply, rebuild and otherwise support your military assets.

Stealth

During the interstellar flight of your factory, you could also launch sneak attacks: Ships launching from the station would not be seen by either your enemies or spies on your homeworld, and preventing communication from a military base in the middle of space would be comparatively easy. You would launch small, hard-to-detect craft and send them off into the cluster of your enemy (not directly at one of their stars), and then either pose as a trader coming from one of their planets or as some rouge rock coming from the opposite direction to where the enemy is looking. If only a few get through, do guerilla stuff; and if you can embed houndreds before the enemy notices, you can draw away their defence force with a feint attack and let the "comet ships" unleash hell on their infrastructure...

Final Option In Case Of Stalemate

You tried for thousands of years, but couldn't get through your enemies defenses? Weaponize one of your solar systems. You start building a dyson swarm and equipping each module with a big laser and accelerate your star using a Shkadov thruster (or a Caplan thruster if you are in a hurry). After 10 million years (Shkadov thruster, 3 ly distance) or 500000 years (Caplan thruster, 8 ly distance) you would reach your enemies planet and, now having built the whole dyson swarm weapons array, could easily dispatch of their forces - assuming they don't have a weaponized dyson swarm of their own. You could also use such a laser array to fire a death ray over distances of many light years, but since movable assets could still dodge - light speed is finite, after all - and you want to take the planets intact, that might be inferior to moving closer and killing the enemy in "melee combat" (<1 ly distance).

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I do not believe victory is possible here. You are describing a technology that permits relativistic kinetic energy weapons. Since the objective is conquest rather than obliteration these would not actually be used, but they would exist as a last resort to ensure you didn't lose. Both sides would have them, therefore neither side could lose, thus you would not have open warfare.

The closest model we have is the Cold War. ICBMs take the role of RKEWs.

You would also see both sides trying to build new colonies unknown to the other to provide survival if the RKEWs ever did fly.

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It will be a massive conflagration of counter-strikes.

The basic problem in your universe is that it takes troops (and in general everything) a couple hundred years to travel between star systems.

Now, as @Flater already pointed out, if your ships go FTL then your crews will experience practically no travel time (or even arrive in the past, but you probably don't want to open that particular can of worms). However, the people they are attacking will have had up to twice the fleet's travel time[1] to resolve their differences. As a result, the invading fleet will have exponentially higher morale than the people they are attacking.

The high moral of the attacking fleet, combined with the inherent problems of defense in space[2], will mean that the attacking fleet will initially be able to do a lot of damage to the world they are attacking. They will probably be destroyed immediately afterwards[3] due to the defenders' technological advantages, but the damage will still have been done.

However, it doesn't really matter how much damage the attacking fleet does. The defenders will have a serious case of Spanish Honor / "Righteous" Indignation, spurring them to launch a counter-strike. When this counter-strike reaches the enemy planet it too will in most likelihood be destroyed, continuing the bloody cycle.

Even if the attackers use planet-killing weapons, it won't make a huge difference; the planet will have time to see its impending doom and spitefully send off a huge fleet.


[1]: The travel time of the fleet itself + the travel time of the fleet due to which it is counter-striking.

[2]: The problem with defense in space which nobody mentions is that space is big. Really big. Furthermore, it's big in all directions; attacking fleets don't have to come in on the plane of the ecliptic, and can split into multiple groups to ensure that at least some of them get through. As a result, defense in depth is more or less impossible.

[3]: Even if they do, by some miracle, manage to win, it'll be just that: a miracle. Attacking fleets do win occasionally, but not enough to stop the cycle.

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At 200 years FTL travel time, that is not a neighboring system. But it may well be a neighboring inhabitable system.

At 200 years travel time, we also probably aren't talking more than one ship every 10 years, and probably more like every 50. So every 25 years a ship arrives, takes out the incumbent government, sets up a government on the other side, and adds some colonists. Within 25 years, the new ship either isn't well functional or departs, just in time for the other side's ship to arrive. After this happens a while, the two sets of colonists make peace and decide they don't want either empire, establish a joint government, and deceive various incoming ships.

Personally, I see this as comparable to England, France and Spain fighting over the new world.

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I would imagine that they don't even try to send ships to attack each other. if communication is near instantaneous, they will each invest heavily in cyber attacks - if you can take over even a fraction of their technology in one of the outer worlds of their solar system, you could use that to build a devastating weapon and they would have much less warning of anything happening. Each side would have amazing cyber security but everyone would also learn to be hackers. The only time they would actually travel between systems would be if they a) already overtook a system remotely and want to inhabit it or b) to send small ships that would get past the enemy radar that future generations could activate to inspect/take over the enemy equipment via rewiring/reprogramming it directly or to build their own weapons on the fringes of the solar system.

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Under the circumstances, I suggest combat by stiffly-worded letters to the Editor, conveyed at lightspeed to the offices of the Intergalactic Times.

I believe there are precedents for letter writers having conducted warfare in this matter for the best part of a human lifetime.

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In addition to some of the other nice answers here:

Propaganda

We (the humans) have been conquered 1,000 years ago by the Aardvark people. They leave us to run our own planet but demand that we provide resources, production and manpower to their empire. Now, look at that fancy instant communications technology*, that means that all-day every day we are getting junk email, robot telephone calls and other messages from the Ostrich empire saying "Hey guys. Are those Aardvarks oppressing you? If you fight for independence then we can protect you from being re-conquered by them? By the way here are some weapon schematics the Aardvarks would prefer you didn't have."

Then, 1,000 years later the messages read:

"Hey Earthlings. We hear the Ostrich people are treating you real bad. Looks like all the weapons they taught you to build just deactivated when they wanted them too huh? We can send data for some new types of weapon. Fight for your freedom!"

Over hundreds of captured civilisations, and some period of time, the better propagandist wins. I am imagining an entire species of super-cute puppy-like aliens appearing on the broadcasts every day telling you how the Aardvarks are committing a genocide to wipe them all out. Of course the cute aliens and the genocide are fictional, cooked up by the Ostriches. But the fake videos are super convincing and it would take 1,000 years to fly to the star system it is supposedly happening in and back to check for yourself.

Maybe a "clean up" fleet needs to be sent to finish off the last star system or two that just likes the Aardvarks too much. But by that point its hundreds of systems vs. 2 s

* We will assume that normal science fiction rules apply and FTL works but for some reason does not provide time travel.

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