@Meta basicllly dude I usually say that even if the Legion did take the dam, which they possibly could do at the second battle, I dont see any way that they can hold it, let alone the entire Mojave. The Legion is a tough as nails opponent but its like the Courier tell Lanius at the end of the game, the Legion cant hold the Mojave and the territory that they've already in Arizona and Utah, their army just cant hold up against that kind of strain. If they do then their army is going to be just as strained in not only the Mojave but all the area they cover and at that point you know the NCR is going to strike back, their not just going to sit back in California and watch somebody else take the land that they've fought for the past five years. They're going to send in more expirenced troops instead of the rookies they've got out there fighting now, and sooner or later they'll take back Vegas and the dam. It may be a long and bloody fight, way worse than the one we've seen so far, but like that moron Oliver says, when the bear is pissed it gets stuff done, or something to that effect anyway.
But that just it the NCR would most likely build some sort of wall and become more fortified preparing for a legion assault because A. NCR has a horrible logistic problem with the divide of death being there. B. Legion would just spam troops from Arizona C. Most of the factions are a. weak or b. with CL. D. during the end game slides legion does end most of the conflict in the Mojave with only the BOS and the Boomers being a problem E. didn't NCR send most of its veteran troops from Baja to Hoover? and finally NCR only wants the dam and the strip not much else because their economy is in the tubes they really do not want the land CL wants the Land but doesn't care about the Dam. Also Ed really thinks that when they take the Strip CL would become something better do to the whole an-thesis thesis speech he gave.
