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
One of the upcoming Apocalypse War Zones is the "Damocles Rematch". It's the Raven Guard and the White Scars up against Sept T'au. The cover image for the novel is a pic of Khan and Shrike kicking Shadowsun's ass (not that anything will come from it, GW would never kill a character even if it would mean advancing the plot even a little). They're also making a big deal about some "Obsidian Knight" guy.
In the fluff, as the battles over the Cadian sector have begun to slacken, battle-hardened imperial commanders are once again turning their glory-hungry gazes towards Tau space. Though the Warp will probably freeze over before GW commissions another Crusade.
Actually, the plot advances and Shadowsun inflicts heavy casualties on both the raven guard and white scars. She even Kills the Raven Guard Chapter master; IN MELEE, allowing Shrike to get a promotion. The imperium is about to send in another crusade to deal with the tau, and Farsight is coming back to help.