The Epsilon crusade was the Imperial effort to liberate the sector of the same name in the Ultima Segmentum, began offically in 999.M41.
The crusade itself was on a similar scale to the previous Macharian Crusade, Sabbat Worlds Crusade and the Crusade of Fire, with hundreds of Imperial Guard regiments committed to the crusade, a reinforced Imperial Navy Battlefleet, detachments from several Titan Legions and elements from several Space Marine Chapters.
The sector began to experience an increase in local dissent with a number of small scale uprisings and revolts in the years following 826.M41. The situation continued to worsen for the Imperial residents and military forces, until, by the year 961.M41, the entire sector could no longer be considered to be under Imperial control, and the Civitas Imperialis was formally suspended.
The task of retaking the sector was eventually given to Lord General Militant Thoren Marbray, who had petitioned the High Lords of Terra for the mission at the successful conclusion of the Yunoz Wars in 998.M41. Marbray was aware of the difficulties the crusade would face due to its location in the Segmentum and the distance between it and the closest inhabited Imperial world, along with the fact that the majority of the sector had been lost in a warp storm for the past 20 years. This did not deter Marbray who was fiercely passionate about reestablishing Imperial Rule throughout the sector. Much like the Crusade of Light had set out to do before it turned into the Crusade of Fire due to the sub-sector already being lost to Chaos.
He had been drawing up plans to retake the sector even before the end of the Yunoz wars, and with their end, the High Lords of Terra granted Marbray a formal charter to reestablish Imperial rule in the sector.
Any jealousy that might have been felt by Marbrays fellow general officers at his revival of Crusade charter was tempered by trepidation of being charged with such an undertaking themselves. The Epsilon sector was comprised of near a hundred inhabited Imperial worlds, covering a vast area of space; the forces of Chaos that occupied these worlds were legion in number and had been fortifying some of these worlds for near a century.
Having gathered the Crusade force on the edge of the sector, where they had been assembling for a year, the crusade began with Operation Lernaean. With almost one billion Imperial Guardsmen, spearheads made up companies of Space Marines from varying chapters, supported on the ground by the Titanic war machines of the Adeptus Mechanicus and supported and carried through space by the Imperial Navy.
The crusade offical began on 132.999.M41 / 0600 HRS, 17 FEB 40999
The inital attack of the Epsilon Crusade was named after the Agrolid Lernaean, a multi-headed serpent that could attack several targets at once with deadly effect.
During the planning phase of the Crusade Lord Militant Marbray quickly determined that it would be folly to invade the Chaos-held sector, which was vast and heavily fortified, along a single front, no matter how overwhelming a force was assembled. Taking inspiration from Warmaster Slaydo's opening move in the Sabbat Worlds Crusade and taking heed from Lord Militant Arian Vois's initial engagement with Chaos in the Crusade of Fire. Instead, Marbray decided to use his ships and soldiers as a mobile strike force, and strike at several target worlds at once. Thus, he split the Crusade force into three Army Groups to strike multiple places at once.
The objectives of Lernaean where:
To secure a foothold in the sector to use as the main base.
To take the enemy completely by surprise.
To prevent enemy counter-attack on the Imperial fleet elements.
To provide a swift and impressive victory for morale purposes.
Over thirty different attack plans were considered and then rejected, by Marbray and his tacticians. While Lord General Kasky advocated a "Mid-Line" approach, Marbray ultimately decided on four target worlds: Apodis, Unicae, Arabis and Rhada.
Apodis was still under Imperial control when the first Imperial forces arrived, and very warmly welcomed as the people had been living in fear of Chaos forces since the warp storm appeared. The system was quickly brought back to the full military operation and converted into the staging system. Unicae fell swiftly to a force commanded by Lord General Kasky, but Arabis was not liberated until the White Scars Space Marines were deployed.
Rhada, the most distant of the four worlds, was considered the "counter-punch" of Operation Lernaean, and its conquest was entrusted to Lord General Mordax Strab. The campaign was protracted, especially with the arrival of Chaos reinforcements at Strab's rear, and lasted well into 000.M42, before Strab ordered orbital bombardment.
Although several battlegrounds, especially Unicae and Rhada, were harder to win than Marbray originally predicted, Operation Lernaean succeeded in all four of its objectives, and morale remained high as Marbray prepared to launch the next phase, the invasion of the Zaria Trailing Worlds, on the path to the stronghold world of Concordia.
However, while preparing, a priority alert was received from a nearby system that was going to be a key supply system for future operations in the sector. In response, the 33rd Army Group was redesignated 41st with additional forces and dispatched to secure the system. However, despite their efforts, the system was infested by Tyranid Genesteelers, and system-wide exterminatus was carried out.
Operation Zaria was the second major phase of the Epsilon Crusade, following immediately from the completion of the first, Operation Lernaean. Its focus was the Zaria Trailing Group.
Lernaean was the initial phase of the crusade, designed to provide a swift, complete victory for morale purposes and give the crusade a foothold within the Archenemy's territory. The territorial gains of Lernaean were a modest four systems. Lord Militant Marbray fully expected Zaria to be a longer and more complicated campaign, not only because its goals were much more ambitious, but also because he expected retaliation after Archon Galnax "woke up" from the surprise of Lernaean.
The Chaos forces opposing Operation Zaria came in three primary forms:
Entrenched planetary defenders, often numbering in the millions;
The counter-attack forces immediately dispatched by Galnax, in the form of raider fleets and convoys to resupply the defenders;
Mobile opportunists, either retreating from Lernaean or having abandoned their posts ahead of the Imperial advance, who, while disorganized, caused plenty of havoc behind Imperial lines.
Zaria's ultimate goal was the capture of the strategically vital world of Chertan.