How to get an unlikely ally to join the NCR in Fallout: New Vegas
One of the best moments in the entire Fallout series is undoubtedly the Second Battle of Hoover Dam in Fallout: New Vegas. The battle is the culmination of every action the player has taken on their journey through the Mojave Wasteland. Players are tasked with choosing on of four sides in the battle: Mr. House, Yes Man, Caesar's Legion, or the New California Republic. Each side has the potential to have allies join them at the battle, with the NCR having the most possible factions coming to it's aide.
The NCR can have every faction they encounter in their main quest join them in some way, and a few who they don't meet in the main quest. The total list is: Followers of the Apocalypse, Boomers, Enclave Remnants, Great Khans, The Kings (guarding Vegas, not at the battle itself), and the Brotherhood of Steel.
Out of all these factions, the most difficult to convince to join the NCR is the Brotherhood of Steel. The Brotherhood and the NCR recently had a war, which can be seen throughout the game. The NCR won, and forced the Brotherhood into hiding in the Hidden Valley.
The Brotherhood is clearly not on good terms with the NCR, but that can change. While completing the main quest for the NCR, Colonel Moore will tell the player to destroy the Brotherhood's bunker. However, this is not the only solution. The Brotherhood of Steel has a quest of their own, Still in the Dark. Depending on how it is completed, the Brotherhood can join the NCR.
Still in the Dark has two endings: one where the current Brotherhood elder, Mcnamara stays in power, and one where Paladin Hardin becomes the elder. It is very important that Mcnamara stays in power, as only he is willing to fight alongside the NCR. If Still in the Dark is completed with Mcnamara in power, he can be convinced to sign a treaty with the NCR, resulting in Brotherhood knights defending the dam from Caesar's Legion.
The New California Republic and the Brotherhood of Steel have a complicated history, but it can be reconciled for the sake of the Mojave. They both desire to destroy Caesar's Legion, and neither wants to continue fighting the other. Despite their spotted past with one another, these two titans of the Fallout universe can find themselves working together.