Iron Cage

The Iron Cage is the name given to the greatest battle fought between the Imperial Fists Legion of Space Marines and their ancient rivals, the Chaos Space Marines of the Iron Warriors Traitor Legion, on the world of Sebastus IV in the early 31st Millennium during the years following the end of the Horus Heresy. The Imperial Fists assaulted the planet after its defences had already been prepared by the Iron Warriors, who had nicknamed their network of fortresses and defensive positions "the Eternal Fortress." The Imperial Fists fell into the trap and suffered horrendous casualties before the Iron Warriors were driven off-planet only due to the intervention of the Ultramarines Legion.

Rivalry
The Imperial Fists Legion of Space Marines, lead by the Primarch Rogal Dorn, and the Iron Warriors Legion, lead by the Primarch Perturabo, had developed an unhealthy rivalry during the Great Crusade. Both Astartes Legions possessed a similar speciality in the construction of fortifications and siege warfare, which had led them to compete against one another in these areas of military science. Tensions between the two Legions escalated dangerously once Dorn and his Legion were given the honour of fortifying the Imperial Palace on Terra by the Emperor of Mankind himself after the Great Crusade reaches it high point with the Imperial victory during the Ullanor Crusade. This marked them as the greatest builders of fortifications in the Imperium of Man. Not long after, the Warmaster Horus declared the Iron Warriors to be the true masters of siege warfare. Fulgrim, the Primarch of the Emperor's Children Legion, once asked Dorn if he believed the Palace could withstand an assault by the Iron Warriors. Dorn replied that the defences he had constructed for the Imperial Palace could throw back any attacker, including the Iron Warriors, so long as they possessed enough defenders to adequately man them. When Perturabo learned what Dorn had said, the Primarch became enraged and responded with insults so vile and baseless against his brother Primarch that many of the witnesses were shocked by the sheer level of vitriol. After this incident became known more widely in the Imperium, the two Primarchs rarely spoke and their Legions never again served together on campaign.

The growing antagonism between the two Astartes Legions expressed itself during the Horus Heresy. After Horus' victory at the Drop Site Massacre on Istvaan V, Horus counted on the hatred the Iron Warriors held for the former brothers-in-arms to carry them to victory and dispatched them to intercept a large Imperial Fists fleet which culminated in the naval Battle of Phall in the star system of the same name. During the siege of the Imperial Palace which Perturabo oversaw during the Battle of Terra, the Iron Warriors took perverse pleasure in violently destroying the defences erected by the Imperial Fists.

Following the end of the Heresy, Rogal Dorn led his Legion against the remaining Traitor Legions to drive them back into the Eye of Terror, though he concentrated his forces on driving the Iron Warriors from the Imperial worlds they had conquered during the Great Crusade and the Heresy.

The Eternal Fortress
Perturabo was determined to retaliate against Dorn and the Imperial Fists for their success in driving the Iron Warriors from their conquests as well as finally demonstrate the superiority of the Iron Warriors in siege warfare. Perturabo believed that the defeat of Horus had robbed him of the pleasure of overwhelming Dorn's defence of the Imperial Palace. To enact his vengeance, Perturabo constructed what he called the Eternal Fortress on the world of Sebastus IV, a defensive trap that was specifically designed to bog down a ground assault by the Imperial Fists and let the Iron Warriors inflict overwhelming casualties on their hated rivals. The Fortress consisted of over 32 square kilometres (20 miles) of hardened bunkers, towers, minefields, trenches, razor wire, tank traps, redoubts and a system of complex underground tunnels. The defenses of the fortress were shaped in the form of an eight-pointed Star of Chaos. At the center of the Eternal Fortress' defences lay a massive command bunker that was intended to serve as a decoy but actually had no tactical or strategic value.

Upon discovering that the Iron Warriors had taken refuge within the Eternal Fortress, Dorn declared that he and his Legion would travel to Sebastus IV and bring Perturabo back to Terra in an iron cage. The Primarch wanted to continue his Crusade against the hated Iron Warriors, but more importantly, Dorn decided to brng an end to his resistance to the Primarch Roboute Guilliman's Reformation of the Imperium, who had called in his Codex Astartes for all of the Space Marine Legions to be broken up into 1,000-man Chapters to prevent the reoccurrence of another Horus Heresy. Dorn had at first deeply opposed the changes to his beloved Legion required by the Codex, but he now saw the wisdom of his brother Primarch's plans. Dorn decided that his Imperial Fists would abide by the Codex, but that the entire Imperial Fists Legion would symbolically undergo a purging test of pain like the Pain Glove that the Legion used on its new recruits and emerge ready and willing to reorganize into the new Second Founding Chapters. The defeat of the Iron Warriors' Eternal Fortress would provide the perfect metaphorical Pain Glove for the entire Legion. As such, Dorn knew that his Legion might suffer heavy casualties, but even he did not expect what happened during the battle that the Imperial Fists would remember only as the Iron Cage.

Orbital Assault
Against the advice of his brother Primarch, Roboute Guilliman of the Ultramarines Legion, that no single Legion could effectively combat the defences the Iron Warriors had erected on Sebastus IV, Dorn launched the campaign with only his single Legion. Even worse, the assault was launched without the extensive planning and preparations that was usually so characteristic of the Imperial Fists. The attack began with an orbital bombardment of Sebastus IV by the Imperial Fists' Legion fleet. However, the Iron Warriors had easily anticipated the tactic and took cover in the Eternal Fortress' extensive underground system of bunkers and remained unaffected by the ferocity of the bombardment. The Traitor Marines then activated remote planetary defence missile silos a sizable distance away from the Fortress, forcing the Imperial Fists to commit their ground forces against them in a precipitous combat drop and ground assault if they did not want to see their fleet chewed up by the world's ground emplacements. As the Loyalist Astartes approached the planet's surface, the Iron Warriors detonated the missile silos in a massive thermonuclear explosion which sent large amounts of radioactive debris into the atmosphere and produced an ionisation effect that disrupted Vox (radio) communications between the Imperial Fists' ground forces and the Legion's orbiting fleet. This tactic eliminated any chance of effective air support for the Imperial Fists' ground assault, a key advantage for the defenders.

The Battle in Space
The destruction of the Sebastus IV missile silos also served as a signal for the hidden Iron Warriors Legion fleet to attack their Loyalist counterparts in orbit. While the two fleets were evenly matched, the Iron Warriors' starships possessed large complements of Chaos Space Marines prepared to engage in ship-to-ship boarding operations while the Imperial Fists ships had despatched almost all of their Astartes to the planet's surface to participate in the ground assault. Using their numerical advantage in orbit, the Iron Warriors managed to scatter the Imperial Fists' fleet, further preventing it from providing support to their ground forces below.

Ground Assault
The Imperial Fists approached the Eternal Fortress' outer perimeter in a massive front a full four companies across. Overseeing the battlefield from a heavily fortified command tower, Perturabo directed the course of the Iron Warriors defence. The Imperial Fists were immediately met by the unyielding and multi-layered nature of the Iron Warriors' defences. With the aid of tank traps and minefields, the Iron Warriors neutralized the Imperial Fists' armour assets and pinned down their infantry units.

The Imperial Fists, despite hig casualties, eventually managed to break through the Eternal Fortress' outer perimeter, only to discover less resistance than expected within the Fortress' lines. Unknown to the Loyalist Astartes, Perturabo had withdrawn portions of his force to entice the Imperial Fists into assaulting the defenders' positions and taking additional casualties. This allowed the Iron Warriors to control the movements of the Imperial Fists, driving them into carefully planned traps and kill-zones. Through the skillful use of these tactics the Iron Warriors were slowly able to break apart the tactical organisation of the Imperial Fists, scattering them by squad and company across the Eternal Fortress' prepared defences where they could not concentrate their force. By the sixth day of the battle, the Imperial Fists had been reduced to fighting a series of individual engagements that could not hope to penetrate the Iron Warriors' layered defences.

The battle lasted another three weeks. The surviving Imperial Fists' ammunition was almost completely expended and they chose to fight in hand-to-hand combat in half flooded trenches using only their Combat Knives, refusing to submit while they dug into the mud and used the corpses of their fallen Battle-Brothers for cover. Rogal Dorn was forced to intevrvene personally a number of times to turn back assaults that would have slaughtered the remaining Fists. The Legion's captains pleaded with their Primarch to organise a breakout and retreat from the Eternal Fortress' lines, but Dorn refused. Ever loyal, the Imperial Fists resigned themselves to die fighting beside their Primarch.

Conclusion
After three weeks and six days, the Ultramarines finally intervened despite the Imperial Fists' protests, driving the the Iron Warriors off the world with a combined orbital and ground assault. Unable to defeat 2 full Legions that had combined their forces, Perturabo concentrated on denying the Imperial Fists' ability to withdraw intact and preventing them from collecting their dead and wounded. Over 400 Loyalist Battle-Brothers' bodies were never recovered by the Legion, and this failure still ranks as a terrible dishonour for the Imperial Fists and all of their Successor Chapters.

The Iron Warriors succeeded in their plan to inflict overwhelming casualties on their hated rivals but failed to destroy the Imperial Fists entirely. The reasons for this failure have long been debated by Imperial historians. One school argues that while Perturabo could have destroyed the Imperial Fists at any moment once they were arrogant enough to fall into his trap, he did not do so because he actually relished Rogal Dorn's anguish at living with his mistake. Another argument claims that the Iron Warriors simply lacked the determination to make the sacrifices necessary to completely eliminate the Imperial Fists.

However, what remained of the Imperial Fists proved to be a hardened, veteran force willing and able to comply with the demands of the Codex Astartes, just as Dorn had intended. Following the outcome of the Battle of the Iron Cage, the Imperial Fists Legion was then split into at least three Successor Chapters, including the Imperial Fists, the Crimson Fists and the Black Templars. All three Chapters would remember the mistakes and the arrogance that had led to the terrible defeat on Sebastus IV and would vow to never again be so foolish. The Iron Warriors escaped into the Eye of Terror following the Iron Cahe and in exchange for forfeiting their captured Loyalist Astartes gene-seed to the foul purposes of the Chaos Gods, Perturabo was allowed to ascend to become a Daemon Prince of Chaos Undivided.