In the Aftermath of the Horus Heresy one of the greatest battles of mankind was fought to determine the fate of a Legion.
The Iron Cage involved two of the mightiest Space Marine Legions in a head-to-head conflict that tested the old question about Unstoppable Force vs Immovable Object. Only in this case, the roles are reversed…
The Iron Cage was a battle fought between the Iron Warriors and Imperial Fists in the immediate aftermath of the Horus Heresy. The Battle involved the entire Imperial Fists Legion and a sizable contingent of Iron Warriors, although the specific size of the latter is not known.
Rivalry and Enmity
The Imperial Fists, lead by Rogal Dorn, and Iron Warriors, lead by Perturabo, developed a strong rivalry bordering on disdain during the Great Crusade. Both Legions possessed like specializations in the areas of defense and siege warfare, leading to competition between the two. Tensions escalated dramatically shortly after Dorn and his Legion were given the honor of fortifying the Imperial Palace on Terra after the successful conclusion of the Ullanor Crusade. Shortly thereafter Horus declared the Iron Warriors to be the true masters of siege warfare, Fulgrim asked Dorn if he believed the Palace could withstand an assault by the Iron Warriors. Dorn answered that the defenses of the Palace would hold against any adversary providing they were adequately manned. Perturabo went into a deep rage, responding with insults so vile and baseless that onlookers were dismayed. Afterward, the two primarchs rarely spoke and their Legions never again served in the same campaign.
The enmity between the two Legions expressed itself during the Horus Heresy, particularly in the conduct of the Iron Warriors. After Horus‘ victory at Isstvan V, Horus counted on the disdain the Iron Warriors had for their loyalist counterparts and dispatched them to intercept a large Imperial Fists fleet, resulting in the Battle at the Phall system. During the Siege of the Imperial Palace, which Perturabo oversaw, the Iron Warriors took perverse pleasure in dismantling the defenses erected by the Imperial Fists.
During the period immediately following the Heresy, when Dorn led his Legion against the Traitor Legions, he concentrated especially on Iron Warriors. In the course of his crusade against the Legion, he systematically dismantled the empire they had developed during the Great Crusade.
The Eternal Fortress
Perturabo resolved to retaliate against his brother for undoing his empire and to demonstrate his superiority in siege after the death of Horus denied him the opportunity to fully destroy the defenses of the Imperial Palace. To do so, Perturabo constructed the Eternal Fortress on the world of Sebastus IV, a mighty trap designed to ensnare the Imperial Fists and allow his Iron Warriors to inflict overwhelming casualties. The Fortress consisted of over 20 square miles of bunkers, towers, minefields, trenches, razor wire, tank traps, redoubts and a system of complex underground tunnels. The defenses of the fortress were shaped as an eight-point star. At the center was a massive bunker intended to serve as a decoy and of no value whatsoever.
Upon discovering the Eternal Fortress, Dorn declared that he would go there and bring Perturabo back to Terra in an iron cage. His determination to do so had two motivations. First, Dorn of course wanted to continue his crusade against the Traitor Legions and the Iron Warriors in particular. Second, Dorn decided to resolve the Codex Astartes Crisis by accepting the Codex but willed that the entire Imperial Fists Legion would symbolically enter into the pain glove together and emerge ready and willing to reorganize into Chapters. Assaulting the Iron Warriors’ Eternal Fortress provided just such an opportunity. This indicates that Dorn was prepared for his Legion to take heavy casualties and perhaps even invited the prospect.
The Battle of the Iron Cage
Despite the attempts of his brother primarch, Roboute Guilliman, Dorn launched the assault alone. Furthermore, the assault was launched without the extensive planning and preparations characteristic of the Imperial Fists. The attack began with an orbital bombardment from the loyalist fleet. However, the Iron Warriors anticipated the tactic and took cover in the Fortress’ underground system of bunkers, effectively remaining unscathed. The traitors then activated remote weapon silos a sizable distance away from the Fortress, forcing the Imperial Fists to commit their ground forces against them in precipitous combat drop. As the loyalists approached, the Iron Warriors detonated the weapon silos in a massive explosion, sending debris into the atmosphere and preventing communication between the Imperial Fists’ ground forces and fleet.
The Battle in Space
The explosion also served as the signal to the Iron Warriors fleet to attack the loyalists in orbit. While the two fleets were evenly matched, the Iron Warrior vessels possessed large compliments of Space Marines prepared to engage in ship-to-ship boarding operations while the Imperial Fists had dispatched their battle-brothers to conduct ground operations. Using this to their advantage, the Iron Warriors scattered the loyalist fleet, preventing them from providing support to their forces bellow.
Assaulting the Eternal Fortress
The loyalists approached the Fortress in a massive formation four Companies across. Overseeing the battlefield from a tower, Perturabo directed the defense against his brothers’ forces. The Imperial Fists were immediately met with heavy defenses. With the aid of tank traps and minefields, the Iron Warriors neutralized their armor and pinned their infantry units.
Upon breaking out, the Loyalists discovered less resistance than expected. Unknown to the loyalists, Perturabo was withdrawing portions of his force to entice the Imperial Fists to assault their positions and take additional casualties. Crucially, this allowed the Primarch of the Iron Warriors to control the movements of the Loyalists, directing them into carefully planned traps and kill zones. By repeating this tactic, the Iron Warriors were able to break apart the formations of the Imperial Fists, first Company by Company and then Squad by Squad. By the sixth day, the Imperial Fists had been reduced to largely fighting Battle-Brother by Battle-Brother.
The battle lasted another three weeks. The Imperial Fists’ ammunition was expended and they fought individually in half flooded trenches with their combat knives, refusing to submit while they dug into the mud and used their fallen Battle-Brothers for cover. Dorn personally turned back a number of attacks. The Legion’s Captains pleaded with their primarch to organize a breakout and retreat, but Dorn refused. Ever loyal, the Imperial Fists resigned to die fighting alongside their lord if necessary.
Ending and Aftermath
After three weeks and six days, Ultramarines intervened, driving off the Iron Warriors. Unable to defeat both Legions combined, Perturabo concentrated his forces on denying the Imperial Fists withdraw and disrupting their ability to collect their dead and wounded battle-brothers, of which 400 were never recovered.
The Iron Warriors succeeded in inflicting overwhelming casualties but failed to destroy their bitter enemy totally. Source material conflicts as to why this was the case, however. One source claims while Perturabo could have destroyed his hated once-brother-Legion at any moment, he did not do so because he so much relished in their torture. Another source claims that the Iron Warriors lacked the faith to sacrifice what was required to completely destroy the Imperial Fists.
What remained of the Imperial Fists was a hardened, veteran force willing and able to comply with the Codex Astartes, just as Dorn had intended. The Legion was then split into at least five Chapters, including the Crimson Fists, Black Templars, Soul Drinkers, and Excoriators.
Only in the greatest challenges do we find true tests of character.