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 pre-War Poseidon Energy 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 the 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 Army forces stormed the power plant in 2276. While Brotherhood troops were able to defend themselves with their advanced weaponry, equipment and power armor, the NCR Army outnumbered them heavily and had the facility surrounded, granting them a significant numerical and tactical advantage. The NCR could afford to lose more troops than the Brotherhood, and was set on capturing the facility. Due to the overwhelming number of NCR soldiers, the Brotherhood Paladins began to sustain heavy losses, making it even more difficult to fight from the 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, he adamantly refused, insisting that he had almost unlocked the secret of the facility: the Archimedes weapon system.
However, Elijah did not anticipate that he would 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 the men and women under his command and disappeared into the wastes. Forced to assume command as Elijah deserted, McNamara wasted no time in gathering the survivors and leading a retreat to 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. The NCR Army had won the battle, though, and inflicted a serious defeat on the Mojave chapter of the Brotherhood of Steel.
The NCR Army was preoccupied with securing Helios One and did not think of launching a second offensive to finish off the Brotherhood until the surviving members had long since disappeared into Hidden Valley. Aware that the Brotherhood and Caesar's Legion were both in the area, the NCR garrison hunkered down and worked to get the power back on. Senior officers like Colonel Cassandra Moore longed for the chance to find and destroy the remaining members of the Brotherhood in the Mojave, but could not find a way to do so without weakening the NCR's position at Helios One and risking an attack by the Legion. As a result, a stalemate developed, with the NCR and the Brotherhood still at war, but effectively unable to attack one another.
With their former 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 lockdown 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 lockdown order remained in effect for five years, until 2281, when 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 had prepared its members for a sequestered existence, the passivity of the current situation proved highly stressful.
The NCR's attempts to restore power at the HELIOS One facility were complicated by the NCR's severe lack of qualified scientists, technicians and engineers. In the process of looking for anyone at all who could work on the facility, an incompetent chem-addict 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 man hired by the NCR would not figure out a way to unlock the facility's dark secrets.