Damocles Crusade
The Damocles Crusade (also known as the Damocles Gulf Crusade) was a major campaign against the Tau Empire by the Imperium of Man.
It began as a response to first contact between humanity and the Tau, during what the Tau called their Second Phase Expansion. The Tau's colonization efforts brought them across the Damocles Gulf, and they encountered human worlds on the other side. These frontier planets were easy to subvert, as they felt little attachment to the Imperium, and even when there were loyalist factions, they were in the minority.
The Imperium was not happy to be losing worlds to Tau rule, so they sent a Crusade to toss them out, and eventually crush them all the way back to their home. They were so serious about this Crusade that they printed a special edition of the Imperial Infantryman's Uplifting Primer just for Imperial Guard Regiments going to the Damocles Gulf. They were initially successful, but as the Tau brought more of their armies to the front, the Imperial advance slowed and eventually halted on the planet of Dal'yth Prime. This was mostly because the crusade had outrun its supply line by many years. You'd think the Imperium would know better than to do that by now. They aren't exactly inexperienced at war. In this case, they underestimated the Tau, who up until that point had only offered token resistance as they tried to evacuate worlds in front of the human advance and buy themselves time to recall their fleets to regroup at Dal'yth where they would make their stand.
Mind you, Dal'yth was a sept-world of First Sphere, meaning it was heavily fortified and there were shitton of fire warriors and auxiliaries garrisoned on it. Imperial fleet sustained heavy casualties even when approaching the planet, and while they managed to destroy or repel all Tau and Kroot spacecraft, literally every battleship of the crusade was damaged rendering the fleet useless for anything but guarding Munitorum transport ships till the end of campaign. Regardless, Imperial forces successfully made planetfall, and on the first day they crushed the garrison forces on their landing zones with relative ease. This was the only thing they got easy on Dal'yth. As war progressed, Imperial forces found themselves constantly harassed by Tau night raids, carpet bombed by thousands of Barracudas and Tiger Sharks, and ambushed by Kroot kindreds. Even with their overwhelming numbers and orbital bombardment support, Imperial forces were tied up by maneuverable Tau forces and unable to move far away from landing zones due to the supply lines being torn apart by stealth-teams. Thousands of Imperial Guardsmen clashed with Kroot and Fire Warriors, while Space Marines were engaged in firefights with Tau battlesuits. Imperial titans found themselves in fights with Manta destroyers, which weren't favorable engagements for them at all, considering most Titan weapons aren't suited against fast flying targets. And yet humans advanced against all odds. Then Tau brought in reinforcements, lead by the now (in)famous commanders Shadowsun and Farsight, and Imperial reinforcements got lost in the Warp only to appear one hundred years later (ironically to help Tau destroy Hive Fleet Gorgon), and the conflict went to stalemate. Stalemated because, now that the crusade's supply line was who-knows-how-many years away, the rest of the Tau military had arrived. Not enough to stop the Imperium, of course, but enough to slow them until their lack of supplies eventually removed everything but lasguns and swords from play.
The Imperium probably would have kept throwing regiments and Titans at the Damocles Gulf, but that was when Hive Fleet Behemoth showed up and they needed every ship and guardsman they had to repel the invasion of Macragge, so they negotiated a cease-fire with the Tau and withdrew. The Tau Empire, shaken by the show of force from a power they thought weak and distant, tried to reassure the people and regain their expansion's momentum by re-taking all the worlds they had lost to the Imperium, and later launched the Third Phase Expansion to take further advantage of the Imperium's retreat.
At the end Tau learned valuable lessons about Imperium's strengths and weakneses, and their own blind spots (such as shitty merchant fleet), which they then patched ASAP. Imperium, being Imperium, learned nothing, making the same damn mistakes with overstretched supply lines and frontal assaults through killing grounds during Nimbosa, Taros and Zeits campaigns, and ultimately losing all of them, despite initial success. However, it is strongly hinted that, despite all their advanced technology and everything they have learned, the only reason the Tau have been largely successful with their Third Phase of Expansion is that the majority of the Imperium's military might has been diverted to defend against Tyranid hive-fleets, WAAAGH Ghazkhull and the Chaos assault on the Cadian Gate along thousands and thousands of other conflicts with a threat level far above those of the Tau Empire. Abaddon's 13th Black Crusade alone has tied up hundreds of guard regiments and dozens of space marine chapters, including almost two full battlefleets; leaving only token forces along with Ultramarines in the Ultima Segmentum.
The Name
Damocles, who the campaign and region was named after, was a man in Ancient Sicillia, who was stern fan of the local tyrant called Dionysios and considered him to be very fortunate. Dionysios wondered why Damocles liked his job and his kingdom so much, and went to teach him a lesson. He invited Damocles for a party at his residence, which Damocles of course was superexcited about - there was only one thing that Dionysios hadn't told Damocles, and that was that, while eating, Damocles would be laying under a hanging sword, attached to the ceiling by a single hair from a horse' mane.
After a very strained and unhappy dinner, Dionysios explained the act to Damocles - He wanted Damocles to feel like he does, like a tyrant. A tyrant always has a sword over him, that can fall down and strike him down at any moment. He is never at peace and can never relax, but can't do anything about it, either: That would just result in him killing innocents and doing henious, but ultimately useless acts. Damocles was scarred for life, but had learnt a lesson. Never again did he talk to Dionysios' praise.
What has that to do with the Damocles Crusade? Well, the Imperium did attack the Tau Empire, a faction notorious for being unassailable by your usual means, and ultimately ended up being humbled by their futile act, while the Tau Empire only grew stronger from the experience...
My god, have they got no clue.
Rematch
250 years later, after dealing with Ork and Tyranid invasions into their territory while fighting small lightning strike wars against Imperium, Tau eventually launched a full scale expansion into Damocles sector.
It started with a series of lightning strike attacks against Zeits sector, to which Imperium responded by sending almost all available guard from nearby sectors and chapter-sized space marine force of dozens Ultramarine successor chapters led by Cato Sicarius. It appeared to be a great Imperial victory, as space marines were able to rout local guardsmen and drove Tau raiding parties out of the sector. In fact it was just a distraction to bleed the Damocles sector off reinforcements and Tau forces (which Sicarius mistakenly estimated as million-sized) were masquerading as entire Third Sphere army while being only a minor fraction of what the Tau Empire actually prepared for war. Way to go being Shadowsun’s unwitting pupped, golden boy!
And then Tau came in force all guns blazing with a goal to take over entire Damocles sector, conquering poorly garrisoned planets in dozens, while Imperial forces were scrampling to retreat and regroup. The first real battle happened on a hive-world of Agrellan, to which most of the fleeing garrison retreated and which was a major logistics node instrumental to taking control over Gilded Worlds (a tightly packed group of mineral-rich planets in the center of the sector). While Tau are generally uncomfortable with sieges and city-fighting, preferring to lure their enemies to more favorable positions, there were simply no alternatives to taking down Agrellan if they wanted to conquer the Damocles Gulf, so Shadowsun took a page out of Farsight's book and planned and executed a perfect massive simultaneous multi-side killing blow assault on Imperial defenses, using all the cheese she could get, including new Riptide suits humans weren't familiar with. Even though they were outnumbered six-to one, Tau scored a decisive victory, at no small part for Riptides serving as a perfect Distraction Carnifexes, Shadowsun’s infiltration force killing imperial command HQ resulting with guardsmen running around like headless chickens, and and one of the new Tau experimental weapons accidentally blowing up a fucking moon. Even while White Scars, Raven Guard and some Knights of the house Terryn came to the rescue, the rape train gain too much momentum and it had no brakes – the only thing marines and knights manged to do is to pierce the encirclement o allow few survivors to evacuate and barely secure LZs to let that evacuation happen, and it costed them. Agrellan was taken over by the Tau Empire and renamed to Mu'Gulath, becoming the new sept world and a base of operations for entire Third Sphere Army.
In an uncharacteristic display of hot-headiness, Shadowsun lead a pursuit over retreating imperials to an insignificant feudal world, but it was a trap and the pursuit force get decimated, though Shadowsun herself managed to avoid space marine hunting force and escape. This caused her to regain her cool and return to her regular cold-hearted calculative bitch behavior.
Following their advance towards the Gilded Worlds, Tau attacked a fortress world of Perfectia, but their old friends of Raven Guard, White Scars and knight were already there and with reinforcements, with RG bringin almost their entire chapter. The war turned into a a contest of cunning traps and escaping them with Raven Guarrd captains and Fire Caste commanders trying to outsmart each other with a limited success for both sides, and thw White Scars being ctched in a crossfire and Kayouned so hard it caused Kor'Sarro to rage so hard that Khorne himleft noticed it. The resulting clusterfuck ended with a massive battle where Shadowsun proved herself a superior trap master (mistress?) and even pulled an Alpharius gambit on the Raven Guard chapter master, blowing him to bits with her new Ghostkeel suit while giving her own XV-22 to a sitting duck volounteer. When the war turned into a meat grinder the deployment of the new Stormsurge suits proved instrumental to fighting off imperial Knights, and when surviving Knights begun their retreat and evacuation, the rest of the imperials broke and retreated too. While marines continued to resist, and Srike even lead a successful counter-attack to retake Severaxes gene-seed, the war was pretty much over. It wasn't as curb-stomp as a Mu'Gulath victory, but despite suffering heavy loses Tau conquered the Perfectia and renamed it Vas'Talos. But while Tau propaganda hailed it as a brilliant victory, it took too much time, and the Imperium unusually hasty prepared launched a counter-invasion on Mu’Gulath, and this time with a shitton of guardsmen who are much better at buying time and turning wars into meat grinders in which Imperium feels hiimself more confident.
Then Farsight drops in. Despite having every reason to shoot him, the blueskins manage to rally to his Enclaves and the Tau finally get to push back again. In response, though, the Imperium decides to employ the Officio Assassinorum Execution Force on four of the top Tau Leaders: Commander Darkstrider, Shadowsun, Farsight, and Aun'Va. While the first three end in some degree of failure or another, the Culexus sent to kill Aun'Va manages to actually kill his prey, leading to the actual death of a named character in....fuck, FOREVER. In order to keep everyone calm, they replace the xeno-pope with a hologram as they decided to fall back. And to put in a final insult, the Adeptus Mechanicus decide to put up a massive firewall and drop the Exterminatus on Argellan (The planet they fought hon) and erect a massive firewall around the Damocles sector, cutting any Tau colonies off from supplies from the empire proper.
In the end, Shrike now has to take on bigger roles, Kor'sarro's still salty about getting owned, and Shadowsun and Farsight decide to agree to disagree, if only because they don't have the manpower to waste on killing each other. And speaking of that red bastard, he returns to find his Enclaves being eaten by Tyranids.
Spoils of war, bitch.