|Conflict||Third Sphere of Expansion|
|Outcome||Imperial tactical victory|
Tau strategic victory
|Captain Cato Sicarius||Commander Shadowsun|
Knights of the Raven
Tau expansionist activities had, in 997.M41, begun to encroach on Imperial worlds, even annexing them on occasion. By 999, planetary governors were calling for massive reinforcements to push back the Tau forces, but with the attacks of Hive Fleet Leviathan and an increase in the attacks from the Maelstrom, few were able to respond. One who could however, was Marneus Calgar. He dispatched the Ultramarines 2nd company to the Zeist Sector to draw a line through which the Tau would not be able to pass. Also, Calgar sent requests to other nearby chapters for further assistance.
When the Ultramarines arrived, they found numerous Imperial worlds abandoned and other heavily fortified to defend against the Tau lightning raids. Cato Sicarius, Captain of the 2nd company, attacked the Forge World Praetonis V to break the Tau assault there and used this momentum to begin pushing the Tau back. Sicarius used the same tactics as the Tau, which were the most ideal tactics for Space Marines, rapid lightning strikes on weak positions to destabilise more heavily defended ones. Quickly, the Tau expansion slowed and then stopped under the assaults. Eventually only the world of Augura 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 the advanced technology, Battlesuits and 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 and the Tau expansion ended with the Tau falling back to protect more secure territory.
Unfortunately, though the Space Marines had won the war, it was not possible to sanction a thrust into Tau held space as the individual Space Marine forces were required elsewhere in the galaxy and it was only reluctantly that Sicarius gave control of the defence back to the planetary governors.
It was later recognized that the retreating of Tau in this campaign was only a trick from the famous Commander Shadowsun who claimed her victory elsewhere where there was not enough of the Imperium forces at that moment.
- Tactical Squads: Night Watch, Halo Dragons, Silver Skulls, Sable Swords, Crimson Fists
- Devastator Squads: Iron Lords
- Predator Spearhead: Aurora Chapter
- Terminators: Eagle Warriors
- Other - Three Companies of Knights of the Raven
- 1: Codex: Space Marines (5th Edition), pg. 48
- 2: Codex: Tau Empire (7th Edition), pg. 42, Relentless Expansion - War Across the Stars