Even though the relationship between the NCR and the Brotherhood of Steel was never without some tensions, it got much worse when Jeremy Maxson became the leader of the Brotherhood. He favored a return to power by wresting all advanced tech from the hands of "lesser people" by any means necessary. Some time after 2242 the war with the New California Republic was announced.
For years the war waged on and was considered a victory for the Brotherhood of Steel. However, no matter how many troops fell to the Brotherhood's superior technology, the NCR always seemed to have more replacements available. The Brotherhood, however, was not so fortunate. Being an elitist group, replacements were short in arriving. It soon became obvious that the Brotherhood was doomed to lose the war to the NCR's greater numbers. Morale at the bunker began to falter as the war seemed more and more hopeless. Eventually the inevitable happened. Lower ranking members of the Brotherhood began to desert their posts.
At some point between 2241 and 2253, Hoover Dam joined the NCR. During the war with the Brotherhood, the Dam lost contact with the rest of the NCR and considered itself the capital city of the Republic. The Brotherhood had taken it over at some point, but the NCR managed to retake it in 2280.
End
I will grant that it is Van Buren stuff but please remember the people who were making van buren, at least many of them, are making this game.
BoS won the battle but lost the war.