Arminius Crusade

The Arminius Crusade was a monumental Imperial Crusade spearheaded by the Iron Spears Space Marine Chapter in defence of the Celtic Sector located in the Segmentum Ultima against the invading Greenskin forces of WAAAGH! Garrog. This Crusade culminated in the First Kilandros Campaign in 780.M40, with the destruction of the Ork Warlord, which broke the back of the WAAAGH!

Background
In 777.M40, the Ork Warlord known as Garrog united several Orkish Klans into a mighty WAAAGH! and invaded the Celtic Sector In response to the Ork threat, the Celtic Sector Authority launched the Arminius Crusade. For two-and-a-half years the forces of the Arminius battled against the rampaging WAAAGH! Garrog until Warboss Garrog was slain and the Ork Klans scattered. Despite the scattering of the Ork Klans, it would take another six months before the Arminius Crusade could retake all of the Celtic Sector.

First Kilandros Campaign (780.M40)
In the 34th year of the Arminius Crusade, the Iron Spears Chapter was able to track down Garrog's own "Mad Skullz Klan" to the arid world of Kilandros and the surrounding systems. Unlike the other Klans of WAAAGH! Garrog which had fallen one-by-one, the Mad Skullz Klan was able to survive and hold off the onslaught Imperial Crusade forces. Even the entire Iron Spears Chapter could not permanently dislodge the Mad Skullz from their stronghold. Every time the Mad Skullz were seemingly annihilated and all their spores destroyed, they would always reappear soon after regardless. After a prolonged siege, the Imperium deemed the First Kilandros Campaign to be a failure. Instead of spending resources on one prolonged assault that would last an eternity, the Imperium would instead launch periodic raids against the Mad Skullz Klan to keep the Ork population under control and prevent them from expanding.