From Halopedia, the Halo wiki
Firebase Bravo was the name of a UNSC Marine Firebase set up on Sigma Octanus IV, Grid 13 x 24, during the Battle of Sigma Octanus IV. It was partially dug into a hillside in a valley to minimize its visibility. The base itself was protected by a ring of sensors encircling the encampment. Radar and motion detectors linked surface to air missiles and the compound itself was protected and surrounded by fixed gun emplacements and barb wire. 
All that changed when the Covenant deployed legions of ground troops, including Grunts, Jackals and Hunters, the latter of which humanity had never previously encountered. Banshee fliers supported the ambush by air. Despite its defenses, the base was overrun. The only survivors of the base were part of a team sent out along with Second Lieutenant McCasky to set up equipment. Four Marines: Corporal Harland, Private Walker, Private Fincher and Private Cochran were all that made it out alive, only to discover that the Marines at Alpha Headquarters had been overwhelmed and slaughtered in a similar fashion as well.
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