The Interstellar Civil War, retroactively named by historians as the Inner Colony Wars, was a large-scale conflict that was fought between the military forces of the United Earth Government (UEG) and the Colonial Independence Bloc (CIB). The Civil War was compromised of the most destructive battles that occurred prior to the Human-Covenant War, with a number of worlds suffering from lasting damage to their industrial, economic, and population centres. It was divided into two smaller conflicts; the larger Central Theatre, which was dominated by bombings and uprisings, and the Gaelic Theatre, the furthest-most strip where the most vicious fighting took place.
Although it is not known whenever the protests should be included, open fighting began in the city of Darian Hills, Cascade, on October 8, 2387. Aided by the knowledge of secret slipspace routes that were never disclosed to the UEG, the complete lack of interstellar communication at the time, and major corporate support, the CIB made significant early gains. A number of colonies declared for the revolutionaries when their cabinets were assassinated or held hostage by armed terrorists, and the garrisons found themselves attacked by the very police forces they worked alongside. The CMA rallied around regional strongholds such as Reach and Paris IV, although the fall of their local area headquarters on Falkirk left their soldiers in the Gaelic Sector isolated. The CMA continued to lose territory until 2389, when a unified plan was put in place to secure and retake their lost worlds, and for the next five years, they would be the only major military organisation to deploy their full might against the rebels.
The UNSC would not enter the conflict until March 28, 2394, when revolutionaries aligning themselves with the CIB attempted to bomb the Prosperity Habitat near Ganymede. Although a large task force assisted the CMA's forces that were operating around Epsilon Eridani, where the two organisations would alternatively take and hold systems to prevent their allies from being overwhelmed. UNSC assistance increased drastically following the suspension of civilian control in 2396, and it was at this point that the Civil War turned against the CIB. Their strongholds in the Gaelic Sector came under attack shortly afterwards, and they surrendered following the capture of their last remaining senior leader, Marta Das, during the Liberation of Estuary.
The aftermath of the Inner Colony Wars saw sweeping changes to the UEG's structure, and to this day there is extensive debate on its far-reaching consequences. The UNSC, previously only permitted to enforce the security of the Sol system, was given the right to patrol the Inner Colonies in peacetime. Thus, this began the beginning of their encroachment on the CMA's rights and authority. In the civilian sector, widespread infrastructure in space was established in the Inner Colonies, and this would eventually pave the way for unified interstellar networks such as the Universal Navigation Array (UNA). However, undoubtedly its greatest and most pronounced result was the beginning of humanity's second wave of colonisation, where refugees were lured onto vast colony ships to escape the overcrowded worlds they have arrived on. This emergency measure accelerated the creation of the Outer Colonies, and the lack of oversight has been argued as being one of the many reasons for why their descendants once again took up arms against the UNSC.