Second Battle of Hoover Dam

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 Hoover Dam in 2282, as well as on the general Mojave Wasteland.

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 and allow the city remain under his control, he had Victor hire a courier to retrieve his Platinum Chip, a data storage device capable of upgrading the city's defense systems and activating a massive army of 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 him/her in the head and leaving him/her 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 his or her 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 traveled the land looking for tribes to ally with, but again, which allied with the Courier, which did not, and which were even outright destroyed is unknown.

Hoover Dam
The Courier's role in the battle depends greatly on which faction they have sided with. If the Courier sided with Mr. House or Yes Man, the battle plays out in a similar fashion, both involving the activation of the Securitron army. From there, the Courier is forced to fight their way across the dam and into the Legion Camp. The NCR quest has the Courier perform the same basic task: fighting through enemy forces to get to the Legion Camp on the other side of the dam. While traversing the dam, any aid the Courier had enlisted will be acted upon by the corresponding faction. Once the Courier reaches the other side of the dam and enters the Legate's Camp, they will face off against Legate Lanius. The resulting confrontation will either have the Courier fight and kill the Legate, or solve the confrontation diplomatically. After dealing with the Legate, the Courier returns to the main gate, at which point General Lee Oliver and his men destroy it. If siding with the NCR, Oliver will congratulate the Courier; however, if siding with Yes Man or Mr. House, the Courier will be forced to confront General Oliver, either fighting and killing Oliver and his men or solving the issue through diplomacy.

If the Courier sided with the polar opposite, the Legion, the battle plays out in a similar fashion to the battle involving the Courier siding with the NCR, only it will playout in reverse; the Courier starts at the Legate's Camp, and then fights their way across the dam to reach the NCR's position, confronting General Oliver. Once again the Courier is forced to choose between killing the General or convincing him to leave without fighting through diplomacy.

Elsewhere
The details of the battle at Hoover Dam are unclear, however the fighting was not just limited to that relatively small location; violence engulfed the entire Mojave Wasteland as Legion forces and its allies attacked NCR targets and locations across the region, with varying levels of success:
 * The NCR Embassy on the New Vegas Strip was bombed, and if they had not been stopped by the Courier already, the Omertas attempted a coup d'état 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 were far more tame.
 * The Fiends attacked Camp McCarran, but if the Courier killed their leaders beforehand, they are easily repulsed.
 * Camp Forlorn Hope was attacked by the Legion, but if 1st Recon were present and the Courier managed to solve the camp's many problems, the NCR soldiers were able to fend off the attack.
 * The Frumentarii 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.
 * Camp Golf was attacked, but the Legion were, due to a heroic effort from the Misfits (if they were trained by the Courier), easily defeated, otherwise Chief Hanlon and the other rangers stationed there were crucified.
 * Novac was attacked by the Legion and many of its citizens were said to have died in its defense, but if the Bright Brotherhood was aided in starting on their pilgrimage, they returned in time to help evacuate the citizens.

Appearances
The Second Battle of Hoover Dam is the finale of Fallout: New Vegas.