The Zeist Campaign was a military conflict fought between the Imperium of Man and the Tau Empire, in the Zeist Sector of the Ultima Segmentum. Taking place in 303.999.M41, during the Tau Empire's Third Sphere Expansion, it ran concurrently with Abaddon the Despoiler's 13th Black Crusade and the assault on Imperial space by the Tyranid Hive Fleet Leviathan. As the Imperium's military was heavily involved with preventing the breakout of the Forces of Chaos from the Cadian Gate on the other side of Imperial space, the Tau took advantage of this distraction to rapidly expand their territory. Despite victory belonging to the Imperium, the Zeist Campaign merely allowed the shrewd Tau Commander Shadowsun to rapidly strike deeply into more valuable Imperial space elsewhere. By the time the Zeist Campaign ended, the Tau's Third Sphere Expansion had already enlarged the Tau Empire by 133% of its prior size. The Tau showed no sign of stopping their aggressive expansion in the Eastern Fringes despite their losses in the Zeist Campaign.
By the year 997.M41, Tau expansionist activities encroached upon Imperial worlds with the initiation of the Tau Empire's Third Sphere Expansion. This Expansion was notable in that the Tau Empire resorted on occasion to annexing Imperial planets by military force, rather than the seemingly more common approach of cultural subversion. 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 attacks by the Chaos renegades from the Maelstrom Warp rift, few Imperial defence forces were able to respond to the call. 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 defensive 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 of 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 ideal for a relatively small force of Space Marines, including rapid lightning strikes on weak positions to destabilize more heavily defended ones. Due to the Ultramarines's efforts, the Tau expansion slowed and then stopped under the Imperial assaults. Eventually, only the world of Augura, in the Zeist Sector, remained in Tau hands, as their only remaining logistical and repair source in the area.
By this time, the Ultramarines were no longer the only Space Marine forces involved in the assault. Warriors 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 large combined force of Space Marines. Augura's fortresses, shipyards and weapons factories were either badly damaged or destroyed by the Space Marines's offensive. With the loss of their last forward logistical base in the sector, the Tau appeared to fall back to protect more secure territory within their borders, apparently bringing their Third Sphere Expansion to a close.
Unfortunately, though the Space Marines had won the war and appeared to stop 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. Reluctantly, Captain Sicarius returned control of the defence of the Zeist Sector back to its Planetary Governors.
Later intelligence would reveal that the Tau had anticipated the Imperial counterattack and that the Zeist Sector campaign was used as a colossal diversion, tying up the elite of the Imperial rapid response forces in a sacrificial zone to allow the Tau to launch offensives against more valuable targets in other sectors, and scythe deeply into more valuable territory. Many worlds of the Imperium toppled to this onslaught. Among the victories won by the Tau at this time was Commander Shadowsun's conquest of the strategically important Imperial world of Agrellan, which took place mere solar months after the conclusion of the Zeist Campaign. Agrellan was conquered by the Tau and renamed Mu'gulath Bay.
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
- Codex: Space Marines (5th Edition), pg. 48
- Codex: Tau Empire (6th Edition), pp. 19, 29