FTL (faster-than-light) travel lane chokepoint

The space battle(s) aren't actually fought about that planet, they're fought because this star system represents a strategically valuable chokepoint in the network of starlanes (or however you want to call them^^).

Starlanes are connections between stars where FTL travel is exceptionally fuel-efficient due to the particular conditions of hyperspace (or some other technobabble). So much so in fact, that most interstellar spaceships can't even carry enough fuel to travel off-lane. Thus, controlling star systems where multiple starlanes converge means controlling easy(ish) to defend chokepoints in the eternal war between (insert space empires and/or rebels here).

Your planet('s star) is located in an area of space which changes ownership every time one of the sides has amassed enough power to push through the defenses the other side has installed there to keep the space behind this chokepoint safe. So it's not one eternal battle, but lots and lots of battles in a seemingly eternal war (because the space empires are evenly matched)

War is peace

https://en.wikipedia.org/wiki/Ministries_of_Nineteen_Eighty-Four#Ministry_of_Peace

The warring nations are locked in an eternal war unable to gain the upper hand because they don't want to gain the upper hand. Both nations are using the war as a means of population control.

As for why they are fighting around this primitive world, it is simply because neither side owns it. In order for the perpetual war to continue, the war must occur far enough away from both nations' populations to keep the illusion of progress while not actually creating any progress. In addition, fighting near one of their own inhabited worlds would mean any collateral damage could tip the scales in the war (which neither side actually wants).

Ritualized, Ceremonial and/or champion combat

Constraints and problems:

Here's the problem. It's hard to have a truly stable state of war with advanced technology and space travel. In an all out war even rocks thrown fast enough are enough to tear a planet to shreds. Any "real" all-out war you'd expect to end in a reasonable timeframe that boils down to when one side can either nuke the other or get into a position where they can credibly threaten to wipe the other out without hope of interception.

It also wouldn't make much sense for combat to happen in one location rather than the front lines moving back and forth.

On top of that, we don't want some stray shots turning the planet below into a burning nuclear hellscape.

We also don't want the combatants landing to build observation posts, radar stations and ground based weapons to aid in the war effort in orbit.

But we want a fairly stable long term war lasting hundreds or thousands of years. Ideally with many factions.

As you say, it would also be nice to have the planet be earth and the combatants be posthumans

We can solve all these issues by making the combat ceremonial

As in ancient times, full scale war is sometimes too costly. When 2 tigers fight, one is killed and one is gravely injured and there are outside threats such that the involved powers can't risk a war that would leave them all weakened.

In those times the leaders would sometimes agree to champion combat.

Or perhaps technology is such that any all-out war between the superpowers of the galaxy would leave all their worlds as burnt cinders.

Thus, when a conflict arises the rules of war dictate that to avoid utter devastation, both sides will nominate a champion or team to take part in ritualized combat. The rules designed to mirror the state of the conflicting parties on a tiny scale.

But where would it make sense for the combat to take place? What neutral ground could be chosen? What system or world would be a suitable location where all can observe the battle and it's outcome but none could lay claim to?

Earth

the ancient homeworld of humanity from which all the galaxies humans descend.
Holy ground where none may land or interfere with the natives.

And so, regularly over thousands of years, small forces from any number of different empires (with different tech including the best each empire can create) arrive in earth orbit and do battle with small quantities of debris raining down.

It may even give you the option of plot lines surrounding the occasional downed pilot.

Space Battles... Naa... Intergalactic Racing!

This planet was a hair-pin turn for intergalactic racing. Nail the gravitational slingshot just right and you're almost assured a 1st place victory - miss it by even a 1/100 of a degree and you're in for a bad day.

Interest in this planet was originally generated when a space (similar to the Voyager) probe was found wandering the outskirts of the known civilized galaxy. The probe not only housed the location of this planet but also a comprehensive bank of detailed genetic models, and many of the models were recreated to build working versions of the planets inhabitants. In particular, life forms were created / modeled after what were believed to be the creators of the probe, and they were later referred to simply as The Probes.

With automation as it was in the galaxy, rather than The Probes being used as slaves, instead they were replicated more as pets and zoo attractions. But as their numbers grew, so too did their desire for independence and desire for self determination. Many died as a consequence of these desires until finally, accords were drafted providing The Probes with a quasi-independent status.

Once permitted a certain level of freedom and the ability to contemplate for themselves a brighter future, many of The Probes were curiously drawn to the past and worked tirelessly to devise a means of traveling to the origin of their dna. When finally they arrived, they found that the planet was barely habitable as it seemed that many hundreds (if not thousands) of years prior, the planet suffered from catastrophic war with many land masses still reflecting high levels of radiation.

Nevertheless, news of the planets (re)discovery sparked wonder in the Galaxy and the planet served as somewhat quasi-homeworld for the genetically (re)created species, so much so that the planet was declared a Galactic Heritage site. It goes without saying however, had the planet been ripe for exploitation (i.e., not adversely radiated), it would have been stripped of its resources, like so many other worlds before it (the planets distance from the Civilized Galaxy also helped keep it safe and in its less than "pristine" condition).

Over time, the interest in the planet waned as newer stories occupied the headlines. One such story was the creation of the Intergalactic Races which would serve as a bridge between the varies alien species throughout the known galaxy. The course would encompass many of the base solar systems for many of the major alien races. The Probes saw this as an opportunity to reignite interest in their newly discovered homeworld and petitioned the Intergalactic Racing Committee to consider their planet as a way-point for the race. For a number of reasons (for example: guilt over their exploitation of the Probes, the somewhat newness of the planets (re)discovery, along with the many other recreated creatures from the dna store), the planet was added to the course.

This segment of the race was by far the most dangerous race checkpoint. It was one of the furthest points from the start/finish line and most ripe for cheating. Many of the most infamous racers were said to employ some sort of nefarious tech to undermine the competition, particularly on the far side of the planet where the flyers were moving at their fastest, barely skimming the atmosphere in hopes of achieving record times. Many ships were lost there, and the losses were generally attributed to "bad" flying - but of course everyone knew (or at least suspected) foul play at hand.

Because of the hazards associated with this segment of the race, the planet was highly monitored to prevent any sort of technology being deployed to the surface - the planet was marked strictly off-limits to any and all alien species. These safeguards helped to prevent alien tech from deployment to the surface which might have provided advantage for some, and detriment to others.

However, tech DID manage to reach the surface - those unfortunate racers "unable" to ride the Razors Edge ultimately found their speeders plummeting to the planets surface: sometimes pilots were able to pod out in time, but many times pilots ended up escorting their fiery craft to the surface - either way, galactic law prevented any and all salvage operations.

Rumors of pilots intentionally crashing their flyers into the planet grew over time, supposedly attempting to deposit illegal tech to be leveraged on future races, as circumstantially evidenced in more recent years of the race where a select number of racers bailed out of their crafts in preliminary races (their flyers spiraling seemingly "out of control" towards the planet surface), but they however miraculously escaping certain death and later in the season, winning Grand Championships.

As such, a growing sentiment for inspectors to visit the planet surface took life, but while the quarantine laws were in place, the probability of inspection was grim (as was the prospect of living aliens interacting with the locals).

The races came to an end by the onset of galactic war, which was brought on by petty trade disputes. Perhaps one day the races may resume, but the isolationist policies that most alien species embraced after the war would need to drastically change.

Flyers have an extraordinary amount of tech. Examples: Communications, Life Support (including reclamation tech for reprocessing pilot waste), Composite Materials, Avionics, Propulsion (with types of fuel), Navigation (including short range Radar), Tracking Modules(used by Race Officials to map flyer movement), Computers, Neural Interfaces, Exotic Metals, Low Density Structures, Batteries, etc.

For each alien species in the race, you can expect one (or more) different flyer makes and models. Assuming you have five alien species flying, it's conceivable that each species brings to the table at least three different models, providing a varying wealth of the above technologies, and all considered Legal tech.

Legal tech, per the unfortunate demise of unlucky racers, was generally salvageable (to a lesser degree anyway) by the locals. If only for ornamental decoration, the inhabitants scavenged this substandard tech to adorn their vehicles, dwellings, etc. Of course some of it too, did manage to survive.

Regards the illegal tech, you can infer flight plans to the planet surface for those flyers was mapped to be less harsh so that the outlaw tech survived more frequently (and was therefore more easily plundered by the locals). Of course when it got scavenged, no one complained since illegal tech "is not" being deposited on the planet surface.

Those aliens unfortunate enough to accompany their crafts to the surface will have implants allowing them more fluid control of their vessels, and it could be harvested and utilized to control the salvaged tech.

Since a host of aliens from different species have crash landed, they bring from their respective worlds: dna/parasites/germs/bacteria/disease/etc. The introduction of alien organisms could account for genetic drift within the local population.

Different alien species also used their unique brand of technology (i.e., different methods of propulsion, etc.), some of which not so friendly to unprotected DNA, and that too caused genetic drift, especially when components of drive systems (for example) were salvaged and used to augment native based drive systems.

Some pilots did survive and built a life for themselves on the surface. Those with a dna signature close enough to the locals managed to mate the locals. The Probes for example were close enough in dna sequencing to permit mating and that also helped in promoting genetic drift.

It is not an "endless war" from the POV of the combatants, but since we are in an Einsteinian universe, spaceflight is limited to STL. Getting from Earth to Alpha Centarui will take 4 years at c, but since you can never reach c, the actual time will be longer. In fact, given the immense amounts of energy needed to reach even substantial fractions of c, the time is much longer.

And that is only between our Solar System and the closest star system.....

Fleets are dispatched according to complicated algorithms since there is no possibility of communication in real time between the fleet and the "home" system. Ships moving at large fractions of c can travel the distance in relatively small amounts of subjective time in their frames, but thousands or even tens of thousands of years pass between the launch and arrival times.

This has several interesting issues.

1. Ships leaving from different star systems will experience different amounts of subjective time. This also means that any new technology developed might arrive before the arrival of the enemy fleet, depending on the geometry of the star systems in relationship to the target.




2. Intelligence will also be disjointed. The news of encounters with the enemy, tactics, technological assessments and so on will also be separated by great gaps in time. If the enemy fleet launched from Alpha Centauri, for example, took 40 years to reach Earth and sent the results of the encounter back via laser beam, the earliest that Alpha Centauri would know would be 44 years later. More distant systems would have this effect exaggerated.




3. Returning crews would be radically displaced in time. They would be decades or centuries removed from their societies at the time of launch, and would have few or no social or personal connections to anyone from their home systems. (Imagine the Spanish Armada left port in the time of Queen Elizabeth I, but is only returning to Cadiz today).

Some of these issues were explored in Joe Haldeman's novel "Forever War", and any part of your story set in space will need to refer to these conditions.

For the people on the ground, the fleets appear at irregular intervals and space combat takes place throughout the Solar System. The occasional "new star" appears and then fades in a matter of hours as ships are savaged with massive amounts of energy. Because of this, space combat is formalized to a certain extent by fighting well away from planets, since they are the ultimate objectives of the fleets. Unleashing a "Nova Bomb" or dropping a Relativistic Kinetic Kill Vehicle (RKKV) on a planet is rather counterproductive in terms of overall strategy.

The gradual rain of metals and other parts of space battles is actually the Earth encountering the debris orbiting the sun (much like the various meteor showers are the Earth crossing the path of ancient comets). There won't be a huge amount of stuff, both because spacecraft encountering high energy weapons will likely be reduced to vapour and metal confetti, and the energy of impact will actually accelerate the debris beyond solar escape velocity: the space junk will leave the solar system forever. So the pieces which do reach the surface are exceptionally "lucky" in terms of the energies being delivered to them, and exceptionally tough in order to survive such treatment.

So even if one side or the other has decided to end the conflict, their fleets dispatched hundreds or thousands of years ago are still on the move, with crews reacting to plans drafted millennia ago and orders they received centuries in the past. For the people on Earth, the war seems have got on for all of history and back into the time of legends, and is likely to continue for centuries to come.

Borderlands

The solar system marks the border between territories of two (or more) rivaling space-faring civilizations. They don't even want to conquer any territory, but they feel the need to prove their technological and military superiority by regularly sending fighter drones to the planets along their borders. It's comparable to earth-based governments displaying their tanks and missiles in military parades with the tiny difference that these drones are displayed in action in a distant solar system no-one cares much about.

It might well be that both civilizations have agreed upon a truce and just continue their quarrels because neither want to be the first to stop in fear of being perceived as weak.

The drones are unmanned, autonomous and relatively cheap, so they're thrown at the opposing civilization without much thought. That's the reason why no-one on the planet ever sees an alien. But they might be able to see flashing lights surrounding other planets (or stars from their perspective) because the fights encompass the whole solar system, not just this one planet.

Religion and tactics

It could also be that one civilization considers this planet the sacred place of their origin. That's why they don't set foot onto the surface, it's too sacred to step on. The opposing civilization wants to conquer this place not for resources, but to lower the morale of their opponents. They send stealth attack squads deep into enemy territory just to keep them busy and divert their resources from the battlefront.

For reasons to fight, see the other answers.

But the Solar System is big. We really shouldn't be seeing much without telescopes, and the odds of any wreckage from some random spot actually hitting the Earth is very very slim.

For some reason, they take the battles to Earth orbit so that wreckage can fall down here.

I would suggest that the reason is the Moon.

The Moon has a nice lack of atmosphere and a reasonable gravity and is generally a perfect place for a military base. There might be other bases around the system but at least one is on the Moon.

Earth itself is not so interesting. Too high gravity and all that horrible air.

And where is a military base, there will be fighting. Attackers will be slingshotting around Earth to strafe the base at high speed. Being hit by the defenders while slingshotting and pieces will be raining down on our poor planet. And missiles missing will have a good chance of going the same way.

You try to see picture so big you cannot see any of it.

Look at planet earth. In the last thousands of years there was no moment in its history that somewhere we hadn't had a war going on. For someone looking from a distance (like different solar system) that may appear as "eternal war on a planet".

You think this war was fought my same multiple races for one sole reason. While in reality there might be multiple wars, fought by multiple races with each other in multiple combinations (just look at Europe in its last 500 years) for different reasons that YOU may narrow into one:

• Some races just didn't want THIS planet to belong to THAT other race.


• Someone's grandma was buried there.


• It's where those religions have their sacred places.


• Lord Emperor lost his magic key for his fiancé's chastity belt when he was passing over that planet and he REALLY needs to find it (but his competitor Emperor Lord try to get the key as well).


• It's a well known planet to have fights because it's barren and no one is in violation of Space Code X5gHd-SD that forbids space battles around planets over class 2.


• They just met there by accident and changing space parameters is really a drag.


• The land on that planet looks like the face of that one other race guy who insulted our King so we need to destroy it (while they try to preserve for the opposite reason).

Let's say the planet is Earth. The "Aliens" all originated from a single space program that propagated. That program included the coordinates of their home planet on the colony ship. But Earth was, due to whatever reason, completely uninhabitable, and would take millenniums to be habitable, therefore it was hidden deep in the code and programmed to be revealed after that time period was passed. This message only served to record the colonists origin and not very detailed.

Due to the time elapsed since they have left and their points of settlement, the different colonies that set out developed very differently, and have lost most of their history or that possibility of other colonies exit. A lot of them however retained the original programming for their colony ship mainframe, due to sentimental value. And this program started showing anomalies after the time had passed, attempting to broadcast, via rather ancient means the planet of origin.

A lot of the future colonists managed to decipher this message, and even after more time, decided to return. Each of these civilizations, so different from one another, arrive to find another civilization surveying their home planet, and therefore thought of them as invaders.

I am hand waving a lot on why they arrived around the same time or why they did not just decide to communicate, as those are very story based.

Could be that space is a hostile place; all systems with habitable planets are valuable resources and are fought for violently, therefore it is natural to attack competitors in sight. Or even if they are the same species, they are too different to share the planet.

Perhaps the first civilization to arrive was surveying the system before making contact, and the second arrived shortly after, before the first could settle; the others started joining in as they arrived, forming alliances or taking their own stand. Think World War, but in space. Either way the situation just escalated, with each civilization preventing the other from making a landing.

Those that had remained behind on Earth, possibly surviving the inhospitable conditions, lost all progress. But humans love tales of hope and these survivors held on to the tale that the colonists will return some day to save them, and these tales soon grew to legends, all having the common theme of lights (ships) appearing in the sky.

You Can't Go Home Again

Not a war, but a series of failed incursions, everyone after the same prize

Consider a distant-future Earth long after the diaspora, where the newly-risen far-flung colonies have rediscovered the ancestral home of all Terran descendants. How do they know that this is that the ancient home of all humanity ("Dirt" or "Earth" or whatever, a la Harry Harrison in the Stainless Steel Rat series)? Because it's protected by an AI-driven robotic defense fleet that makes this clear to all who approach: exo-humans are not welcome here. Only registered Terran citizens may land. (But of course, no such planet-side citizens still exist.)

The Diaspora

Perhaps at some point in the distant past, most Terra-bound humans were cast out to the stars as a population control measure to save the home planet for the lucky few, and the orbital automated defense systems were designed to enforce that decision. After many millennia, the civilizations of Terra fell into ruin, maybe having finally exhausted their resources and trashed their environment after all, still squandering what they were striving to protect through the overpopulation expulsion program. Access codes were lost, key personnel died, or the AI simply stopped listening to what appeared to be attempts from within to subvert its mission. For whatever reason, there's now a low(er)-tech human presence on the planet and a high-tech robotic presence in space, maintaining and upgrading itself, harvesting resources via asteroid mining operations, etc. - but staying off-planet because that was part of its original charge: that and to destroy all possible incursions from without.

The TOOLS for the Job

Such incursions finally come (or finally come again) - at first, in the form of robotic probes and then as scouts, both from some of the newly-risen-again, far-flung human worlds. As all such missions vanish, small exploratory strike forces arrive to investigate with force, discovering that the Terran Orbital Defense System (TODS?) or Terran Orbital Outsider Lockout System (TOOLS) or (insert other nifty acronym) has technological prowess well beyond the various human worlds' technological capabilities. Though most of those fleets are destroyed, too, inevitably someone escapes to report back, and new incursions are planned, new technologies developed to aid in the fight - but since information on the TODS / TOOLS / ? is so limited, all battles so far have been lost, with much salvage to be enjoyed by the robotic fleet, but with occasional incidental debris falling planet-side, too.

Waves Against the Wall

As word spreads, the human worlds, each with their own degree of cooperation or isolation from the others and thus their own unique designs and technologies (and, perhaps, mutations / engineered genetics / degree of alien-ness), continue variously to send exploratory fleets, then larger and still larger battle fleets, in an attempt to claim the planet and its presumable technological treasure trove. Occasionally, perhaps, they might arrive concurrently and end up battling each other, but (as interstellar travel takes so much time) most often the fleets arrive separately and simply run up against the TODS / TOOLS / ? resulting in more space junk donated to the planet and its unique scavengers.

The Vault Is Empty... ?

Ironically, never having been close enough to gather intel on the planet's surface, none of the exo-human civilizations yet realize that if they ever manage to breach the defense system, only a great big ball of Road Warrior wasteland awaits. But some of the more clever among its denizens may be the key to unlocking an as-yet unseen treasure from all this trash...

There's another civilization, much more powerful than any of the warring parties, that has announced an ultimatum: no one harms or sets foot on that planet, or we are going to go to war.

(why, oh, why, would such an advanced race protect such an unimportant planet - wink wink nudge nudge cough it's because it's Earth and they're from there cough)

They're advanced enough and surveying enough that a false flag operation would be too dangerous to try.

There are complicated loopholes. The occasional accidental gear falling down is fine, but keep the mass lower than X, energy lower than Y, and don't go after it. Colonizing the Moon around it is allowed, but no damage beyond Z amount of energy, no messing with its orbit, and no mass addition or subtraction beyond W kilograms. All in the name of avoiding affecting the planet.

Unintentionally, they made it very advantageous to have a base on that Moon. A logistics center that the enemy cannot just nuke. All sides are constantly trying to set up such bases, and constantly preventing the other sides from doing so, or destroying their bases - the hard way. No one can afford to just let their enemies have the advantage of a Moon base.

The war rages on for a long time for reasons unrelated to this local anomaly. While the war lasts, all sides keep reinforcing their position in this system, as they cannot afford to leave it.

Flower Wars.

The Aztec waged ceremonial war to please their gods and get worthy enemies to be sacrificed. If they fought the war to its grim end they would run out of worthy enemies, tributaries and, above all, worthy sacrifices to the gods. The starfaring civilizations are space Aztecs that wage war for religious reasons - capture warriors to sacrifice. They all descend from an ancient group of earth colonists that left in some sleeper ship and are now coming back in FTL ships. Or maybe it's STL ships but then they will probably be cyber Aztec space elves, that were once Man, to live the thousand years needed to travel across the stars. The captured warriors are taken to Mars and sacrificed in a blood altar there. The solar system is chosen because it's holy. They don't resettle earth because it is holy. They also, due to their religion, take care that no relativistic bullet, antimatter bomb or quark-gluon plasma accelerators hit the holy world.

Based on the Star Trek Voyager Episode Blink of an Eye, where the locals also began worshipping the sky-ship, there could be a time dilution field based on some highly localised tachyon flux (babble babble). Thus, the battle does not actually last ages, but the time on the planet goes much faster. Thus, every 1 minute of space battle an entire year passes on the planet.

Two nations/ space civilisations could battle for the resources in the Systems Asteroids.

From time to time they battle, deploy miners, gather, and get beaten by the other nation and so on.

That would make a lot of battles, lights, junk and the miners won't go down to the planet.

War Games

Several allied spacefaring races, each descended from Humans, return once every few years to their shared world of origin for a friendly competition to test their newest military inventions against each other.

Since you want something like a Mad-Max/post-apocalyptic world, go with a post-apocalyptic universe.

A highly advanced race/civilization created the technology for FTL travel and spread this throughout the universe as part of their vast "empire." This empire was overthrown/destroyed by their subject races (my preference, but some other SciFi trope of your choice would work as well), so it is no longer present in the universe (at least as it once was).

The process to create the FTL "drives" (or some other component required for FTL travel) requires factories that are built in close orbit around stars with very specific compositions (size, age, elements present, etc). The actual means for building these factories and/or their operation (i.e. make them automated) have been lost when the advanced empire/race lost its place in the universe. As such, all the remaining races/factions using FTL drives vie for control over the factories to maintain their position in the universe.

As the centuries have passed, fewer and fewer of these factories remain operational (malfunctions/wear and tear, damaged/destroyed by solar flares/conflict over control/collision with other objects, etc). This makes control of any of the remaining factories a highly desirable objective for any race and they change hands frequently (potentially multiple times per year).

This meets the requirements of the original question and provides a framework for more detail in the following ways:

• Control of at least one factory is key to a space faring civilization's survival. Without access to FTL drives, a civilization loses the ability to repair or build FTL ships. As a finite resource, this will cause repeated conflicts by multiple races/factions.


• Races/factions may be able to claim the "prize" but ownership will be continuously challenged by other races/factions. Even if controlled only briefly (weeks or months) by a race/faction, they would have access to the FTL drives produced during that time.


• Over the centuries, reduction in the number of factories increases the amount of conflict over the remaining factories. This creates a world where you can increase the amount (and types) of materials that make it to the surface over time.


• While races/factions share a common FTL technology, all other components of their civilization/culture could be unique - weapon tech, defensive tech, aesthetic/design choices, entertainment, etc.


• Equipment/tech could come from more than just "warships" as there would likely have to be a supply of resources going to each factory and the completed drives would need to be transported elsewhere to be used to repair or build FTL ships. This opens up raids on supply lines and piracy as sources of technology.


• The nature/type of equipment/technology that makes it to the surface of the planet could change over the centuries as the races/factions struggling for control of the factory changes over time.


• The planet could be Earth; one that was subjugated or nearly wiped out during the reign or fall of the advanced civilization. Races/factions descended in some way from the original inhabitants of Earth would likely form a number of the groups vying for control as they originated from the same galaxy/area of the universe.


• No need to interact with the primitives. If the planet has no resources necessary for the operation of the factory, there is little reason to actually visit the planet. With control of the system changing so frequently, there is also little reason to invest in any sort of infrastructure on the planet.


• There could even be some sort of "Geneva Convention" that all races/factions adhere to that provides "rules" for conflict in systems with factories to prevent further risk of loss due to conflict. These rules could provide the basis for any number of specifics of the races'/factions' interaction in the system that you desire.

When I hear "endless battle" I think of a runaway AI problem.

So you build an AI, you cant program its moral code of conduct as thats far too prone to problems so you have it learn moral codes. You put it in a simulated world and test a trillion moral dilemma's, big and small on both large and small scales for years and years to make sure there is no edgecase that could give problems. You give the AI the task to terraform the Galaxy for any expanding lifeform while supporting any lifeforms it comes across or that develops before sentient species start inhabiting the planet. These species must first reach a certain technological threshold (or doomsday scenario) before the AI lets itself be known and lets other species mingle with them. Then you give it a bunch of self-replicating robots as hosts and send it to the stars.

This code of conduct is basically the AI's main DNA, and since you dont want the AI to rewrite this code of conduct that you approve off on a whim you set it in stone as a set of rules it cannot change ever. Like DNA the data can slowly degrade, be misread or written down wrong on whatever is used to store it. There will be dozens if not hundreds of failsafes to ensure any errors are found and either rewritten properly or completely removed, IE destroyed.

Given enough time (milennia) and enough replicating bots, there will be enough simultaneous errors that a cancer will develop, or in this case a large section of self-replicating bots will be written with a wrong code of conduct and assume its the right one, then try to rewrite this code on all bots that do not have their new code of conduct or destroy them.

The actual difference in the code of conduct can be minuscule and irrelevant to the original owners, in fact all the species they are protecting will likely not even know it if they met bots from the "new" AI or not or landed on a planet terraformed by one or the other. When the new and old AI components meet, they will try to rewrite eachother and when that fails they'll try to destroy the others to prevent the "wrong" code of conduct to spread any further. Since they are simultaneously trying to murder the other but also fulfill their duties they'll be around planets with sentients a lot, as they dont trust the other AI to fulfill it according to their code of conduct. They'll actively try to protect the planet and destroy the opposition. This means stray hits with WMD like weapons are unlikely as it screws with their COC too much, and since the AI has to make sure none of the ships are carrying sentients of another race before they blow it up they will be fighting in much closer combat than you would expect to be better able to spot lifeforms.

The planet is actually stuck in a spatial anomaly. While the inhabitants of the planet actually see nothing wrong, with their sun rising and setting every morning, and the stars crossing the sky at night, in fact the entire planet is trapped in a spatial anomaly. This is perhaps caused by the crystalline core of the planet, which generates something similar to a magnetic field, but much more powerful and gravitometric in nature. Any ship that comes too close to the planet will find itself trapped in a buffer zone between 300 and 10,000 kilometers above the planet's surface. Attempts to come any closer will destroy the ship, while ships attempting to go further away encounter a gravitometric inversion in which the force of gravity increases exponentially. Were a ship to manage to get further than 20,000 kilometer from the surface this inversion would entirely be undetectable, while in the 10,000 to 20,000 kilometer range a kind of "sticky trap" exists where ships are pulled in and can only escape with quick thinking, enough fuel, and some luck. Likewise, between 100 and 300 kilometers the anomaly also tapers off, leaving those on the planet completely unaware. In the middle of the disturbance is actually a calm zone in which a ship might remain for an extended period with only minimal use of fuel for life support.

Any ships unfortunate enough to become trapped in this anomaly quickly find themselves cut off from all radio contact from outside the field. They might perhaps signal optically, but this serves at best as a warning to keep other nearby ships away, as those ships who have attempted a rescue have wound up trapped themselves.

Any ships exploring too close to this planet find themselves in a conundrum. Having managed to avoid destroying themselves try to land and retaining enough fuel during any escape attempts to stay around the planet for a while, conditions are soon found to be dire. A captain might try to team up with any other survivors in the anomaly, or might instead try to attack others and take their resources to prolong survival for a bit longer. Newly trapped ships might find themselves fresh meat and forced to fight for their survival or form alliances as quickly as possible.

The planet is near enough to galactic traffic to occasionally get new ships trapped in its web, and these ships sometimes battle for weeks until the dominant ship or faction of ships is victorious and gains enough reserves to last until the next arrival. But being far enough from standard shipping lanes, the anomaly isn't well known to be avoided or officially investigated either - perhaps with only rumors which serve to attract adventuring types and keep the cycle alive.

If the planet is Earth far into the future, there is no need for conflict. The flashes and debris are simply satellites crashing into each other and deorbiting.

The Prime Directive

The Prime Directive,prohibiting its members from interfering with the internal and natural development of alien civilizations, is an important guiding principle for a major advanced civilization but mostly useless if they let other civilizations interfere.

Turns out that this planet has vast natural basins of liquid H₂O, largely unused by the native sentient race. The second most advanced civilization out there is very eager to get its hand on this resource, while the most advanced civilization is just as eager to protect it.

Thus, you have an everlasting wave of attack ships trying to break through an everlasting defense formation, all while the strongest party makes sure that no contact is made with the planet below.

Does it have to be battles?

The remnants of earth happens to sit beneath Trash Wormhole #3's exit node. Various intergalactic species use this wormhole to dump old and defunct ships as well as waste that is unsalvageable. Small trash burns up on re-entry but sometimes bigger pieces of trash land on this desert wasteland.

Millions of years ago most of earth's inhabitants left on huge ships (think Wall-E) leaving behind only a few who have been forced to live on what is now a barren planet due to global warming/nuclear fallout. Far flung human species now use the solar system as a literal garbage dumping ground which would explain the random appearance of seemingly alien technology and why nobody would ever go there.

A fairly simple solution (ignoring complications of kessler syndrome) would be that your planet is off-limits to the galactic community.

One of the races in eternal battle are essentially the guardians/galaxy police, ensuring that no contact is made. Much in the same way that there are international laws on earth now, regarding the contacting of uncontacted tribes.

The opposing force, then, is like a galactic church sending missionaries to enlighten the locals. They are never very effective, so the guardians always "win" and beat them off.

A Symbol of Status

As you say the planet in question is the ancestral home of all species now existing across the galaxy. And they know it.

Earth - the mother - is the spawn of all civilizations. It is each species' origin, and thus each species seeks to prove their superiority over all other paths of evolution by claiming and settling/preserving/whatevering her.

This leads to an eternal war, ever-fed by new spaceships, stations, experiments in the ultimate race/war/conquest to rise above the others.

We come, because everyone else is already here.

Time has become meaningless, there is nothing left. Literally nothing - Every star in the entire universe has burnt out, exploded throwing it's own mass out into an infinite darkness. All the spacefaring races know the end is soon, but they also know that the only power left, is that which is held by those they may of once called friends, and now have become mortal enemies.

In his final days of exploring, as the universe was literally dying around him and the various species in the universe were all fighting in their home galaxies, or in the places they considered the most strategic and valuable - which is amusing considering the only value now is energy, which is largely lost anyway he made a discovery which blew his mind. A single planet which some how had survived yet never been discovered, untouched by any hand of intelligence. It was pristine and it was unburnt by that which now pervaded all else - the thirst for energy.

He made his choice there and then, jettisoning the energy capsule to land on the planet, meaning the planet would be protected, yet fought over until either there remained one winner and they could collect their winnings, or all would be gone and the universe could finally sleep. His ship came down, far enough away from the energy capsule to ensure that it survived even if he didn't and in his final moments he smiled. The energy capsule was more powerful than some of the largest stars, it'd leak heat and energy into this planet, keeping it living for thousands upon thousands of years if undisturbed and if the wars in the heavens did finally end that too he guessed, made it a fitting prize to whoever could land and take it. But how to make them come? His beacon would let them know, screaming out into the nothingness 'I was here'.

Explanation:

The universe is dead, all the stars are gone - the only source of power left is energy capsules, which are slowly running out. Those races which managed to escape to space all need energy to survive. Somehow, someone ended up finding a planet, after everything was dying that was undisturbed. They launch their energy capsule into the planet, ensuring it's short term survival, but also dooming it to become the largest target in the universe. A huge amount of power inside a planet, inhabited by those who have no clue what they have. All the races of the universe will flock to try and win the battle, to take this energy - mean while on the planet the inhabitants see, but don't understand hat is happening in the heavens above them.

The Moon is an Advanced Outpost

It doesn't matter if the moon is an advantageous strategic position or just the home of some key research facility - the point is that it should be relevant.

So relevant that other alien races (or other versions of humanity, to meet your criteria) keep continuously sending troops to try to reach whatever lies in the surface of the satellite.

You could even use some of the other answers in this very question as seeds to better improve this explanation. For example, Syndic's FTL highway concept is pretty awesome. Why not make it so that the moon is also a pit stop for travelling ships? It would also be an explanation as to why this base can keep receiving enough reinforcements to keep fighting for a very long time.

Building upon optimisticOrca's answer around multiple colony fleets coming back to the earth thousands of years later.

Instead of saying that all aliens(ancient humans) arrived at the same time it could be that there is a large defence satellite array around the earth that has been defending it waiting for the colonists to return however as so much time has passed the array has malfunctioned/ does not recognise them as human and attacks the fleet. As more fleets arrive to a hostile environment they could mistake the defence array as an attack from the other fleets. Over time the defence array could be destroyed but the fleets would continue to attack to defend their so called "holy land" from the alien invaders/ occupiers.

As a way to keep any of the races from gaining an upperhand you could always say the defence array was the pinnacle of human engineering designed to repel any attack and leave it in place so anyone who got near the planet was destroyed. This would stop anyone from claiming the planet as regardless of the size of the fleet it would not be able to get to the surface.

Toxic atmosphere

Though this planet has organic life it has evolved a little bit differently than in other places. At some point bacteria on this planet started producing a toxic gas that through the eons has slowly been building up in the atmosphere and eventually reached a stable level millions of years ago.

Life on the planet has evolved to tolerate this toxic gas as the concentration in the atmosphere was slowly growing. But so far no multi-cellular organism from any other place has been found to survive that gas.

It's so toxic that even with the best available protection anybody who attempts a landing would die before reaching the surface. This is something that may have been learned the hard way.

Exploration and tourism

So who would have an interest in this toxic environment? Civilizations at this level of development may be rare so there is a scientific interest in studying the civilization from orbit.

Additionally this is a popular destination for wealthy tourists who want to see the most unusual sights on their vacation.

Space junk

Over time a lot of space junk has started orbiting this planet. Some of the junk is deliberately deorbited and is responsible for some of the flashes in the sky as larger parts burn up on entry but some parts survive the landing due to being smaller, more robust, and or better protected.

Accidental collisions between junk and orbiters as well as between orbiters and other orbiters are responsible for other flashes in the sky and more space junk of which some may fall to the ground.

Piracy

There is a number of pirates orbiting this planet as well. Likely these have started out as low salaried employees of the various missions to the planet. At some point they may have staged a mutiny and taken control over one or more orbiters. Unlike official missions the pirates don't receive supplies from home, so they have to steal everything they need.

Little did this piracy do to stop tourism to this planet. The tourists came to see the exotic and dangerous planet and space pirates just added to that. Travelers to this planet believes that orbital mechanics is predictable enough that they can spot and evade the pirates. Though over time pirates have found that abducting wealthy tourists for ransom is a profitable business.

Taxation

Various governments have decided to tax travel to this planet. Officially it's to protect the civilization on the planet from evasive tourists, unofficially it's just a source of income. This has lead to wars over who is allowed to collect taxes. Though these wars are of little significance to tourists who are wealthy enough to pay taxes to every government that consider themselves entitled.

Could this be Earth?

I haven't stated which toxic gas is in the atmosphere on the planet. If it was Earth the prime candidate is oxygen. Life in other places may have evolved in a way that did not involve an oxygen rich atmosphere and simply cannot tolerate it. And humans travelling in space may have developed life support systems that supply just the right amount of oxygen directly into the blood stream. Over time evolution has gotten rid of superfluous organs such as lungs, and space craft have replaced oxygen with pure nitrogen if not a complete vacuum. As these human descendants have not been exposed to oxygen for many generations their skin has evolved to a form that is more robust against radiation but vulnerable to oxygen.

Meanwhile civilization on Earth may have experienced major setbacks due to climate change. And survivors many generations later only know fractions of what happened in the past from word of mouth.

Once a war has gone on long enough, it tends to continue just because the other guys have killed / are killing our guys (see most of the world). Provided neither side has a serious chance or the political will for wiping out the other side (easy to posit due to effective planetary defences), the battle continues, away from both sides' home territory.

So it will be in 'no-man's land'. Which means in space the battle tends to continue where you find the other guy, which is where he's looking for you, which is where we've always fought, which happens to be over this random planet.

For bonus fun, the primitive civilisation is actually descendants of early escape pods which crash-landed on the planet generations ago.

Too Much Backstabbing

There are far too many factions fighting each other for any one of them to beat all the rest. And all of them would prevent anyone else from claiming a total victory, at any cost. Nobody can be trusted to share the prize. Therefore, whenever one faction looks like it might win, the others don’t rush to join the winning team; instead, its allies will switch sides to keep it from getting too powerful. Since no one faction is powerful enough to fight a coalition of the others, no one has ever been able to end the war.