United Eridanus Space Force

The United Eridanus Space Force, or UESF for short, is an independent faction that is separate from the UNSC. Formed by the Insurrection in 2562, the UESF has a combined fleet of about 600 ships, whereas the UNSC has a combined force of around 50,000 ships; both stats are from ONI's 2568 fleet estimates. 
Made up of mostly human-built vessels, with at least one CAS-class Assault Carrier; they have an above average naval force, but not enough to fully combat the UNSC, Covenant Remnant, or the Volgath Swarm. They also build their own vessels out of wrecked ships, both Covenant, and Human.
Owning around eight to twelve human colonies along with a few ONI bases and Orbital Defense Platforms, actually conquering this faction is a challenge, as the guerrilla tactics they use are enough to confuse even the best strategists around.