Dela III is a planet in the First Rim, controlled by the UNSC since its colonization in 2500 by the UNSC Galileo. Up until mid-2586, the planet was notorious for rebel activity, especially Insurrectionist regimes. These rebel assaults first began circa 2510. The planet's activity temporarily ceased in 2534 when a Covenant Whisper arrived in system and attacked the three major towns via orbital bombardment resulting in the deaths of 80% of the planet's population, a total of 72,000 people.
The colony was abandoned and remain dormant of UNSC activity until January of 2584 when a Alaska-class Superfreighter accidentally jumped into the system due to a malfunction in its shipboard AI. Contact was quickly lost with the ship, leading the UNSC to assume that the Covenant had a presence in the system (this hypothesis was supported by the fact that the Second Great War was raging on and the UNSC was still a spectator). A fleet of thirty capital ships was assembled and assaulted the system, only to find that Insurrectionist ships, the strongest being two weary HC-War era Frigates, were in system. The ensuing battle, known as the First Battle of the Dela Dellus System, ended in an outright UNSC victory. The UNSC returned continually during mid-late 2585, continuing to destroy Insurrectionist strongholds until May of 2586, when the majority of the system was rebel-free.
After rebellions were quelled, the UNSC re-colonized two planets of the system, Dela V, an oceanic planet rich in deuterium, and Dela III. In 2588, Fort Cabello was established, containing 1,000 Army personel, 400 Air Force personel.