Zeist Campaign

The Zeist Campaign was a conflict fought between the military forces of the Imperium of Man and the Tau Empire, during the Tau's Third Sphere Expansion in 999.M41, in the Zeist Sector of the Ultima Segmentum. It was fought concurrently with the Imperium's involvement in the 13th Black Crusade of Abaddon the Despoiler, but was contested on the opposite end of the galaxy in the Ultima Segmentum's Eastern Fringes. The Tau took advantage of the Imperium's weakness while most of its military forces were concentrated around the Eye of Terror to rapidly expand their territory. By the end of the Zeist Campaign, the Tau Empire had grown to 133% of its size in 997.M41 before the Third Sphere Expansion had begun.

Overview
By the year 996.M41 Tau expansionist activities had begun to encroach upon Imperial worlds when their empire's Third Sphere Expansion was initiated. The Tau Empire even chose to directly annex Imperial planets in their path on occasion. By 999.M41, Imperial planetary governors in the Eastern Fringes of the galaxy were calling for massive reinforcements to push back the Tau forces, but with the constant attacks on Imperial space by the Tyranid Hive Fleet Leviathan and an increase in the attacks of the Chaos renegades from the Maelstrom Warp rift, few Imperial defense forces were able to respond. One Imperial defender who could, however, was the Ultramarines Chapter Master Marneus Calgar and his Space Marines. He dispatched the Ultramarines' famous 2nd Company to the Zeist Sector to draw a line through which the Tau would not be able to pass. Calgar also sent requests to other nearby Space Marine Chapters for further assistance.

When the Ultramarines arrived in the Zeist Sector, they found numerous Imperial worlds abandoned to the Tau and others heavily fortified to defend against the Tau's lightning raids. Captain Cato Sicarius, commander of the Ultramarines' 2nd Company, attacked the Forge World Praetonis V to break the ongoing Tau assault on that planet and used this momentum to begin pushing the Tau offensive back. Captain Sicarius used the same tactics as the Tau, which were the most ideal tactics for a relatively small force of Space Marines, including rapid lightning strikes on weak positions to destabilize more heavily defended ones. Quickly, the Tau expansion slowed and then stopped under the assaults. Eventually, only the world of Augura in the Zeist Sector remained in Tau hands.

By this time, the Ultramarines were no longer the only Space Marine forces in the assault. Forces from the Night Watch, Halo Dragons, Silver Skulls, Sable Swords, Crimson Fists, Iron Lords, Aurora Chapter, Eagle Warriors and the Knights of the Raven, as well as many others, had all joined the Ultramarines in the attack on Augura. Despite their advanced technology, battlesuits and plasma weaponry, the Tau had no hope for defeating such a huge combined force of Space Marines. Augura's fortresses, shipyards and weapons factories were destroyed or badly damaged and the Tau Third Sphere Expansion was halted, with the Tau falling back to protect more secure territory within their borders.

Unfortunately, though the Space Marines had won the war and stopped the Tau's seizure of human territory, it was not possible to sanction a retaliatory thrust into Tau-held space as the individual Space Marine forces were required elsewhere in the galaxy to hold the line against the Forces of Chaos during the height of the 13th Black Crusade and so it was only reluctantly that Captain Sicarius gave control of the defense of the Zeist Sector back to its planetary governors.

Space Marine Forces Involved

 * Tactical Squads - Ultramarines, Night Watch, Halo Dragons, Silver Skulls, Sable Swords, Crimson Fists
 * Devastator Squads - Iron Lords
 * Predator Spearhead - Aurora Chapter
 * Terminator Squad - Eagle Warriors
 * 3 companies of Knights of the Raven