First Battle of Hoover Dam

The First Battle of Hoover Dam was likely the first major engagement between the NCR and Caesar's Legion. The NCR was forced off of the dam by a larger Legion force but retreated into Boulder City. There the NCR Engineers rigged the entire city with explosives and decimated the Legion force as they advanced into the city. The main force of troopers then proceeded to force the Legion back across the dam.

Prelude
The NCR likely had little to no warning about the Legion attack, possibly explaining why there were few troopers stationed at the Dam itself at the time.

There was also a large presence of engineers in the area at the time, and a substantial amount of explosives present. This is what probably enabled the engineers to rig Boulder City to blow at such short notice.

Skirmish at Dam
The small force present at the Dam was likely undermanned and definitely caught off-guard. The legion likely forced them off of the then-unfortified dam in less than an hour. Remaining forces, including present engineers retreated along the US-93. Their retreats were covered by Rangers and 1st Recon, who gave the troops and engineers enough time to settle into defensive positions and rig the majority of Boulder City with explosives.

Rangers and 1st Recon
The Rangers and 1st Recon harassed the Legion troops very effectively along the US-93, where rough terrain and cover provided easy lines of sight and points to shoot at legionnaires and officers from.

Nevertheless, Legion forces made it to Boulder City in fighting shape, though demoralized, delayed, and reduced by snipers along the US-93.

Last Stand
NCR troops were still present in the city at the time of the explosion, engaging the Legion vanguard to allow for the remainder of NCR forces to withdraw and to force the Legion force to amass inside the city. This proved to maximize the damage made on the NCR's enemy, though it killed many of the NCR's force as well.

Explosion
When the time finally came, Engineers detonated the entire city. The Legion's entire vanguard was destroyed, along with a good fraction of its general force. Graham was forced to retreat back east along the US-93.

NCR Initiative
Within hours the NCR's force, now reinforced by railroad-bound troops and supplies from McCarran, was harassing the Legion's rearguard on their retreat back to the dam. Graham likely meant to try and dig in at the dam, but the tactical situation proved too adverse for the Legion to fall back quickly enough to set up a defense.

Aftermath
Joshua Graham was forsaken by the Legion and thrown into the Grand Canyon burning. Hanlon was once more celebrated as a war hero for the Rangers' valiant actions during the Legion's advance toward Boulder City. The Legion's front line receded back across the Colorado. The NCR held Willow Beach and the Arizona Spillway for a short time until it was taken back by the Legion.

The dam was, from then on, held as a much higher strategic priority than just a power plant. A much larger force was also stationed there from then on.