Legion lost the first battel for the Dam. Legion use a nuclear dirty bomb to take out a town. NCR has a very large area to cover and political reason NCR does not send out enough troops. Military Leader wants most troops at the Dam and they a proven right because Caesar attacks the dam. If Caesar was a smart man he would hit NCR where they are not. GO around the Dam and just take out the Strip or Mccarran.
The Legion never used a Nuke, they just opened some radition barrels that where inside the Firehouse; and the Legion has just as much area to cover as the NCR. Confining all you're soldiers in one small area is a bad idea, since you're weakening your flanks.
And Caesar obviously goes around. I mean, Nipton, Cottonwood Cove, Seachlight and Nelson aren't close to the Dam.
And Epicness, Centurions carry Marksmen Rifles, Hunting Shotguns and Anti-Material Rifles aswell; and having a Veteran Ranger get killed would be a Morale hit aswell, since most soldiers consider to be them invincible.