The Nimbosa Crusade
When hive fleet Kraken began to threaten worlds on the Eastern Fringe of the Imperium, the High Lords of Terra responded by stripping the worlds on the border with the Tau Empire of Imperial forces. The Tau Empire was quick to recognise this redeployment and soon moved to take advantage, beginning the period known as the Second Sphere Expansion. The Tau had soon annexed a whole swathe of Imperial worlds along the border. Nimbosa was one of the most important of these worlds.
The Tau forces under the command of O'var (Commander Brightsword), began the invasion with a landing of overwhelming numbers. They soon controlled most of the world and started a systematic extermination of the populace.
First Phase: The Defence
The only Imperial force left able to resist after the first phase of the expansion was the Vostroyan Firstborn IX under the command of Graf Toschenko, who moved quickly to fortify the Factory city of Polia. Brightsword's first thrust used Kroot to identify Imperial Guard strong points to be bombarded by the Tau armour. Kroot auxiliaries suffered heavily in the push but the strategy was successful and the Tau eventually gained a foothold in the city which then began to grow. Graf Toschenko had come to realise that the Tau policy of isolating and reducing each stronghold would soon break his hold on the city, so he authorized a counterattack, the main blow of which fell around the central Cathedral.
At first the counter-attack was a complete success. Kroot auxiliaries were caught by surprise and reeled from the blow, falling back towards the Tau armour. Graf Toschenko committed his last reserve of men and his few remaining tanks to exploit the gap made around the Cathedral. However, the discipline of the isolated Tau forces held, and the salient turned into a trap. The spear tip and reserve was decimated by ambush and bombardment directed by stealth suit teams spotting for artillery and guided munitions fired from Tau rear areas. At the vanguard of the advance, Graf Toschenko's command squad was caught out in the open and destroyed by hidden Tau Fire Warriors. The Graf took up the regimental standard and desperately called to his men to rally. Imperial resistance hardened around the Graf and the fight degenerated in bloody and close street fighting, but by that point Imperial resistance was all but broken and fighting finally ceased with the death of Graf Toschenko.
Second Phase: The Crusade
The initial battles with the Tau took place in space aboard the orbitals the aliens had locked in geo-stationary orbit around Nimbosa. The Black Templars' Strike Cruiser Dorn's Wrath used its speed to evade the incoming fire and close to attack range. Using boarding torpedoes and Thunderhawk gunships it penetrated the hull of the target, designated Orbital Primus, and allowed the household of Castellan Folkert to board.
The Tau Fire Warriors stationed abourd Orbital Primus turned the corridors of the space station into killing grounds with a warren of deadly ambushes, but the Black Templar force countered this by splitting up into small squads and attacked the defenders from multiple directions, using grenades and chainswords to smash their way through the bulkheads. This enabled them to outflank the defending Tau forces, who were pitiful at close combat, and crush the resistance.
After fighting their way through to the gunnery controls of the orbital, the Marines were able to turn the guns of the station on the other orbitals. This, combined with the assaults of the Imperial Navy, was too much for the remaining orbitals to withstand.
Meanwhile, the Ultramarines first company smashed into the defending Tau garrison, eliminating the Tau's anti air turrets and making short work of the Tau leaders. The campaign ended with the successful recapture of Nimbosa and the destruction of the entire Tau garrison.
There seems to be some discrepancy in the timeline for the Nimbosa campaign. Codex: Imperial Guard (5th Edition) places the date for the attack as 790.M41. Cities of Death states, however, that the reason for Nimbosa being lightly guarded was a result of Hive Fleet Kraken, a Hive Fleet whose earliest signs were not seen until 250 years after the destruction of Hive Fleet Behemoth in 745.M41. To add further confusion, Codex: Tau Empire (4th Edition) states that the Imperial response only took four months to enact, while the short story "Assault on Nimbosa" places the counterattack post-999.M41. Also, Codex: Space Marines (6th Edition) dates the Nimbosa Crusade during 993.M41. 
- 1: Cities of Death pgs. 56, 57
- 2: Codex: Imperial Guard (5th Edition) pg. 22
- 3: Codex: Tau Empire (4th Edition) pgs. 14, 15
- 4: Assault on Nimbosa (Short Story) (The contextual clues mention the Ultramarines' recent loss of Uriel Ventris and Pasanius from the ranks of the 4th Company)
- 5: Codex: Space Marines (6th Edition) p. 45.