The Shadow Crusade
During the Horus Heresy, the full might of the Word Bearer and World Eater Legions were unleashed upon the 500 worlds of Ultramar as Horus wished to keep the Ultramarines from interfering with his advance to Terra. Many of the planets in Guilliman’s star empire were subjected to ferocious and unexpected invasions. With almost half of the Ultramarines Legion killed or put out of action after the Battle of Calth, and interstellar communication/travel cut off from one another due to the Lorgar’s ruinstorm, each planet was left to fend for itself. Some planets were able to repel the invaders completely. On Astagar, a force led by Lucien Corvo (later the founder of the Novamarines) managed to win the battle for the planet by destroying a possessed Warlord-Class Titan belonging to the Word Bearers.
On others, the Ultramarines destroyed their worlds rather than let the enemy have them. Such as on Percepton, where an outnumbered chapter of the Ultramarines subjected the planet to phosphex bombardment, burning away both defenders and invaders who were unlucky enough not to evacuate in time.
More traitor legions would also invade Ultramar space during the Heresy, although they were not part of the Shadow Crusade. For example, the 25,000 strong force of Night Lords that were one of the six splinter fleets escaping the Dark Angels would attack Sotha seeking to obtain the Pharos device for themselves. This attack would be repelled and the Night Lords nearly wiped out thanks to the efforts of Warsmith Dantioch, Captain Polux and the company of Ultramarines stationed on Sotha at the time.
In the outer reaches of Ultramar, the Death Guard would also launch probing attacks on the relatively undefended border worlds.
More stuff from my wiki entries to 1d4chan :3 I thought I’d compile a history of Ultramar while I’m at it.