Fans of Fallout: New Vegas know that the Hoover Dam plays a huge role in the game's story. Not only is it the location of the game's final mission, but it's also a crucial resource that the major factions of the Mojave want. While the NCR remains in control of Hoover Dam throughout the game's main story, the eventual ownership of the Dam depends on whom the player sides with during the Second Battle of Hoover Dam, at the end of the game.
The Second Battle of Hoover Dam is a crucial moment in history for the Mohave, though many players may not pick up on what happened during the First Battle. This initial conflict between the NCR and Caesar's Legion sets the stage for pretty much everything that happens during the events of the game. All the information regarding the First Battle can be found by talking to various characters throughout New Vegas and its various DLCs.
The NCR Strategically Took Control of the Hoover Dam
As part of their attempt to spread into the Mojave, the New California Republic took the key strategic resource that was the Hoover Dam. The Dam was both a defensive location and a way for the NCR to provide water and electricity to their citizens. Initially, they were able to secure the Dam and hold it without much issue. The Dam was able to survive the apocalypse thanks to Mr. House's laser defense system, though it wouldn't be properly maintained until the NCR's occupation.
While the NCR initially had little issue taking possession of the Dam, their continued expansion throughout the Mojave greatly weakened the Republic. While the NCR successfully took HELIOS ONE from the Brotherhood of Steel, their continued expansion really began to cause strain on their hold. The repetitive major battles left them in an incredibly weak state -- one that would quickly be taken advantage of by Caesar's Legion.
Caesar's Legion Decided to Make a Play for the Dam
Ulysses, the antagonist in New Vegas's Lonesome Roads DLC, was the one who initially reported the existence of the Hoover Dam to Caesar. He recognized that the Legion had a great opportunity to take the Dam, which may have been enough of an advantage to push the NCR out of the Mojave for good. The Legion began attacking NCR-owned settlements in order to cause as much sabotage to the Republic as they could.
The NCR's hold on the Mojave would grow even weaker when the Courier unknowingly caused a nuclear explosion in The Divide. As a result, one of the NCR's major trade routes, the 127, was no longer usable. With low supplies, thinly-stretched manpower, and continued Legion sabotage, the NCR was beginning to look weaker by the day. Caesar chose Malpais Legate, otherwise known as Joshua Graham, to lead the first attack on Hoover Dam.
How the First Battle of Hoover Dam Played Out
As The Legion began its direct attack on the Dam, General Oliver of the NCR opted for a defensive play. He ordered his forces to fall back to the middle of the Dam where they could easily entrench themselves and hold off the Legion. The Legion appeared to be trying for a quick victory wherein they would crush the weakened NCR. This may have worked if it was not for the NCR Ranger snipers who were set up on a nearby ridge.
The Rangers were ordered to pick off Legion veterans. As they began to fall, the less experienced fighters quickly succumbed to chaos. The Legion had a standard military formation where their newest recruits would lead a charge with their veterans bringing up the rear. This strategy was likely meant to wear down the opponent with sheer numbers before having the veteran soldiers finish them off. However, the NCR was too deeply entrenched to wear down, which quickly began to damage Legion morale.
Still, the NCR likely couldn't hold Hoover Dam forever. They needed to force Malpais Legate to call a retreat. So, they led the remaining Legion veterans into a trap at Boulder City. The Ranger snipers fell back to the city in order to force the veterans to pursue them. When the Legion arrived, however, they found that the Rangers had already retreated and that the city was laced with explosives. This final blow to the Legion was enough for Malpais Legate to order a retreat. The NCR was able to keep ahold of the Hoover Dam.
The Aftermath of the First Battle of Hoover Dam
Following the First Battle of Hoover Dam, Malpais Legate was considered to be a failure in Caesar's eyes. To make an example out of his former friend, he had Malpais covered in pitch and set on fire, before casting him into the Grand Canyon. Against all odds, Malpais survived and would go on to be known by the local tribes as The Burned Man. Legate Lanius was then made Malpais's replacement. Instead of making another attempt at the Dam, Lanius chose to strengthen the Legion further by claiming more nearby tribes.
The NCR, while victorious, still felt the effects of the conflict. Around 100 NCR soldiers died in Boulder City alone, which doesn't even factor in the town's destruction. While pursuing the Legion as they retreated was considered, the idea was dropped due to the difficulties that would arise in reinforcing a front-line assault. Instead, the NCR turned their focus towards trying to secure their territory in the Mojave and prepare for future Legion attacks.