In the Halo universe, the United Rebel Front is the bane of the UEG and UNSC's existence (prior to the Covenant). As their name implies they are a massive unification of rebel and terrorist groups hellbent on getting the so called outer colonies independence from the Inner colonies. According to Halo lore, the URF was created in 2491 when a UNSC naval commander (illegally) used nuclear weapons to put down a rebellion on the colony Far Isle. The URF uses mostly old, outdated weapons and vehicles bought off the black market, though they occasionally steal or capture weapons, equipment, vehicles or even ships from the UNSC. They have tried to overthrow the governments of at least several colonies including Eridanus II, the homeworld of John-117 (aka the master chief). They fight with guerilla tactics, but almost never really win battles against the UNSC without extreme losses. When the UNSC met the Covenant in 2525, the URF became target practice for the alien invaders and for the SPARTAN-II super soldiers.
The United Rebel Front suffered heavy losses during the liberation of Eridanus II.