Armageddon
Armageddon is an Imperial Hive World in the Segmentum Solar. It is one of the most well-known hive worlds in the galaxy, with an industrial output surpassing that of many forge worlds. As it supplies dozens of worlds with materials and protection, Armageddon is a strategically vital world and is heavily garrisoned. Climate-wise, Armageddon is an ash-filled poluted hellhole. On the main continent there is little more than ash filled deserts, though it's southern regions and the secondary continent have jungles. It is home to the Steel Legion, one of the most recognized Imperial Guard Regiments in the galaxy and Armageddon was also the location of some of the largest wars in the history of the galaxy, including the Third War for Armageddon which was the largest land and space battle in the history of the Imperium of man. Notably it was also the largest gathering of Space Marine ships in history, including during the Siege of Terra or the Ullanor Crusade.
First War for Armageddon
The Beginning
The first war for Armageddon began with a series of Chaos rebellions in the hive populous as the demon space hulk Devourer of Souls came out of the warp above the planet. Much of the World Eaters chaos space marine legion was aboard the hulk, and they made planetfall amid the bloody rebellions, slaughtering all resistance on the main continent. Realizing that the main hives were lost, what was left of the loyalist populations on armageddon retreated to the smaller continent across the sea to regroup and resupply at the small military bases and outposts located there. For a few short weeks while the Khornate hordes busied themselves with sacrificing civilians to summon daemons to bolster their ranks, and the Loyalists used this time to fortify as well as they could, but they had little more to offer their foes than flashlights and insults. As the Blood God's hordes turned their eyes to the last defenders, now bolstered by daemons and the Daemon Primarch Angron himself. The ragged refugees and soldiers could do little more than pray for deliverance when...
The Cavalry Arrives
Out of the fucking blue with no real lore explanation the Vlka Fenryka showed up, led by the Old Wolf himself! The wolves blasted the fuck out of the Devourer of Souls and made planetfall to help the remaining Planetary Defense Force and Steel Legion forces defend the remaining civilians while they awaited the help that Logan had called for. After months of bloody battle, the Grey Knights fucking finally showed up to the party. This 'ragged brotherhood' fought Angron and his retinue of Bloodthirsters and won when their brotherhood's Grandmaster was able to banish Angron after Hyperion shattered the daemon Primarch's blade. Of the 109 Grey Knights who took to the field, 13 survived.
The Months of Shame
Following Angron's banishment the Wolves and Steel Legion were able to destroy the few Khornate forces that remained. Given that the whole planet had been contaminated by chaos, and by the sanction of an inquisitor lord the remaining population of Armageddon, civilians and soldiers alike, were to be sterilized and sent to inquisitorial compounds to be worked to death, and any space wolves who had scene the Grey Knights were to be mindwiped, save for Grimnar. The space wolves were not fucking amused. Logan politely and calmly explained to the inquisitor that by punishing the citizens of Armageddon for their heroism and survival, the inquisition were sullying the honor of the men that yet lived and disrespecting the memories of the dead. The inquisitor more or less told Logan to go fuck himself. The Old Wolf was not amused. Shortly after this exchange the refugees and soldier survivors started to mysteriously "disappear" from inquisitorial custody. The inquisition pretty quickly figured out was up and called the wolves to explain their deeds, but they weren't feeling talkative. The inquisition began firing on any civilian craft leaving the planet, including the ones escorted by the space wolves who blocked the inquisitorial fire, first with their void shields and then with the hulls of their ships, but never fired back. After months of tense standoffs and passive resistance against the inquisition, the Space Wolves finally got sick of the inquisiiton's shit when they destroyed the relic Battle Barge Scramsaex and the space vikings started shooting back. Evidently the inquisitor lord thought it was a good idea to be in a void war with Logan Grimnar a dude who can call on EIGHT battle barges and FUCKING THIRTY strike cruisers. He was wrong.
The End
After months of the inquisition getting its ass kicked the Inquisitor finally called on every resource he could muster and set off to Fenris with an entire Imperial Navy fleet, a Grey Knights strike cruiser and the entire Red Hunters chapter fleet. To make a long story short the wolves lost some ships and some men, the inquisition lost a lot of ships and men, and everything ended with Logan Grimnar teleported onto the Grey Knight's strike Cruiser, killed the inquisitor and Grandmaster, and threw Hyperion a beating, but was forced to stand down when Bjorn teleported up into the party and had what is quite possibly the single most "Bjorn" moment ever. Not only did he tell the inquisition to get off his damn lawn, not only did he tell Logan to quit making so much damn noise with his out-of-hand party, Bjorn actually came onto a Fenrisian inquisitor, calling her a "beautiful frost-born maiden" (or some other flowery shit, it's been a couple years since I read The Emperor's Gift, okay?) revealing himself to be a krachety old romantic neckbeard. Everyone was pretty fucking dumbfounded, and the inquisitor was dead, so everybody pretty much went their own way.
TLDR: yiffs fought the insquistors, it was a draw.
Second War for Armageddon
To make a short story even shorter, the Second War for Armageddon was a massive fucking meatgrinder full of happy orks and pissed off humans. Ghazghkull Mag Uruk Thraka arrived on the planet with a bunch of Roks and a few billion Orks and decided to make things more interesting. The local PDF's big cheese was a fucker by the name of Herman Von Strab, though he sucked at his job so Commissar Yarrick, the Old Man himself, took over and started getting shit done. Yarrick managed to hold things together with his half-whipped and half-starved forces until the marines arrived to help out. Commander Dante arrives with a taskforce to finish the fight. Ghazghkull Mag Uruk Thraka fucked off to space. Kinda like WWI, all this war did was set the scene for the big war that came after, which brings us to...
Third War for Armageddon
The third war for Armageddon was the biggest land war in the history of the Imperium and the galaxy's mosh pit. FUCKING EVERYONE was there. Well. All the orks and humans anyway. Seriously, all 9 former SM legions were there along with a lot of successors, there were a dozen titan legions and a metric fuckton of guard regiments, along with a few billion orks. There were more Void Ships and ground forces at the third war for Armageddon than any other war in the history of mankind, surpassing even the Siege of Holy Terra and the Ullanor Crusade (though this has been partially retconned by our overlords). Commanding this clusterfuck you had High Marshal Helbrecht in charge of the Void Battle, as the Templars are void specialists and Commissar Yarrick in overall command on the ground (says a lot about how hardcore Yarrick is that even the Astartes were willing to take orders from this guy). Short story shorter Ghazghkull was annoyed that he had to fuck off to space last time, so he came back with even MORE orks to come back and get Yarrick, though this time the Imperium was prepared and eventually won. Ghazghkull fucked off to space again, with Yarrick joining the Templars on an epic roadtrip to go smite the green fucker. Helbrecht had intended to build Armageddon as a bastion of Imperial might, including having several space fleets that would allow the Imperium to attack and reclaim planets that they had lost, but as soon as he and Yarrick left the lazy fuckers left in charge just did what they had done before the Ork invasion, fuck all and hope nothing happens. Then GW realized that they were terrified of story progression, so this victory was retconned away.