The Second Battle of Hoover Dam is the colloquial name applied to the second offensive mounted by Caesar's Legion against New California Republic forces entrenched on the Hoover Dam structure in 2281.
Prelude
After the First Battle of Hoover Dam, Mr. House knew that as soon as either the NCR or the Legion became victorious at the Dam, they would turn their attention to New Vegas. In an attempt to stop this, he had Victor hire a courier to retrieve his Platinum Chip, a data storage device capable of upgrading the city's defence systems and activating a massive army of PDQ-88b securitrons, which would give House the ability to single-handedly crush the Legion and intimidate the NCR into a retreat. However, the leader of the Chairmen, Benny, learned of this plan and ambushed House's Courier outside of Goodsprings, shooting them in the head and leaving them for dead in a shallow grave and taking the Platinum Chip, throwing House's plans into disarray.
The Courier survived, however, and began a journey to the New Vegas Strip to find Benny and get revenge, in the process learning the true purpose of the Platinum Chip and its power. The Courier was faced with the choice of either giving the Chip to House or destroying the securitron army, or even using it to create an army of their own. The Courier's journey attracted the attention of the NCR and the Legion, both wanting such an influential individual on their side, although which side (if either) the Courier chose remains lost to history. In preparation, the Courier travelled the land looking for tribes to ally with, finding the Boomers, the Brotherhood of Steel, the Followers of the Apocalypse, the Great Khans and the Enclave Remnants, but again, which allied with the Courier, which did not, and which were even outright destroyed is unknown.
The Battle
The details of the battle at Hoover Dam are unclear, however fighting was not just limited to that relatively small location as violence engulfed the entire Mojave Wasteland. The NCR Embassy on the New Vegas Strip was bombed, and the Omertas attempted a coup but were stopped by the Chairmen and the White Glove Society, as well as the securitrons. Rioting broke out in Freeside (although its severity is unclear; if the Courier forged an alliance between the Kings and the NCR the riots are much more tame), and the Fiends attacked Camp McCarran, but again if the Courier killed their leaders, they are easily defeated. Camp Forlorn Hope was attacked by the Legion, and Legion spies made an attempt to bomb the monorail outside McCarran, whether it was successful or not depends on whether the Courier uncovered their plot or not, and at a location known as "Bravo Bravo Charlie", a doctor arrived.
Aftermath
Appearances
The Second Battle of Hoover Dam is mentioned in Fallout: New Vegas.