The War for Bellum
Classified as a Civilized Imperial world, Bellum is not very different from Terra or Cadia. the temperature is slightly cooler than that of Terra. It has few things that seem to make it special or stand out. It is however in a strategic location. Bellum, if taken, can become the launching point for major offensives in the Spinward Front.
The planet Bellum was taken during Saint Drusus campaign that formed the Calixis Sector. A man and future saint named Hathborne, fought a long and brutal war against the Eldar. Hathborne became a hero and upon his death, was entombed at the planetary capital Victoria.
When Severus the 13th declared his secession from the Imperium, Bellum followed. However many citizens who remained loyal, began to use guerrilla tactics on the Severan Dominate. The planet was already in the grips of all out civil war long before the Imperial Guard ever arrived. But the Imperial Guard did arrive, and quickly began to retake the planet. Unfortunately their efforts were stalled by the sudden invasion by Orks.
And now the planet that was once a beautiful Imperial world, has a new classification. Only War.