They'd be marginally more effective. We lost most of the chapter defending glorified artillery.”— Veronica Santangelo
The leader of the Mojave chapter of the Brotherhood, Elijah, ordered troops under his command to seize HELIOS One, a Poseidon Energy pre-War solar plant, after acquiring information about a possible weapons system hidden in or connected to the facility. The Brotherhood presence at the facility eventually drew the attention of New California Republic Army, which had begun deploying to the Mojave en masse to secure Hoover Dam and New Vegas.
With the establishment of a regional stronghold at Camp McCarran, NCR command decided to seize Helios One to both secure U.S. Route 93 and procure another source of electrical power in the Mojave. Although the power plant was hardly defensible, leaving Brotherhood forces exposed to attack on all sides (save a small canyon leading to Hidden Valley), Elder Elijah ordered his troops to dig in and protect him as he attempted to unlock the secret of the facility.
NCR forces stormed the power plant in 2276. While Brotherhood troops were able to defend themselves -- thanks to their tech weapons and power armor -- the numerical and tactical advantage the Republic's army possessed soon translated into casualties among Paladins. Due to the overwhelming number of NCR soldiers, the Brotherhood Paladins began to sustain heavy losses, making it all the harder to fight from a poorly defensible position. Although Head Paladin McNamara repeatedly asked Elder Elijah to call for a tactical retreat in order to save the lives of his men, the latter refused, insisting that he had almost unlocked the secret of the facility, the Archimedes weapon system.
However, Elijah did not anticipate that he'd be unable to use the orbital weapons, as he did not possess Euclid's C-Finder, the tool necessary to aim and fire Archimedes II. The only device he had access to was the previous generation orbital defense system, which was useless as it was preset for test firing within the confines of the Helios One perimeter.
Frustrated, Elder Elijah abandoned troopers under his command and disappeared into the wastes. McNamara wasted no time in gathering the survivors and retreating into Hidden Valley from Helios One, but not before reactivating the pre-War security measures inside the solar collection tower to prevent the NCR from picking up on Elijah's research.
With their leader gone, McNamara assumed command in accordance with the Brotherhood's chain of command protocols. In order to prevent an accidental disclosure of the location of Hidden Valley, their sole remaining stronghold in the Mojave, the new Elder declared a full lock down in effect. Activity outside the bunker was drastically restricted: only supply runs, occasional reconnaissance missions and night patrols were allowed to leave the underground installation and even then, they were strictly forbidden to engage NCR Army units. The lock down order has remained in effect for five years, until 2281 a broken air filtration system forced the Brotherhood to either commit themselves to destruction or become active once again. Though the Brotherhood's ascetic lifestyle has prepared its members for a sequestered existence, the passivity of the current situation has proved highly stressful.
The NCR established a firm foothold in Helios One and its vicinity, although due to a stroke of bad luck, an incompetent con artist was hired to act as the facility's principal engineer. He was unable to bring the facility to even moderate operational status, leaving it at 1% efficiency. Ignacio Rivas was assigned by the Followers of the Apocalypse to independently research the facility and ensure that the fool hired by the NCR would not figure out a way to unlock the facility's dark secrets.
Operation: Sunburst is only mentioned in Fallout: New Vegas.