I think wait and see Oliver is mostly to blame for the situation stagnating as much as it did post first battle for the dam. It was clear after the first battle that there would be a second. Oliver wanted a massive concentration at the dam at all times. You talk to people like Ranger Jackson or Lt. Hayes and they tell you the problems they are dealing with could be solved with just a few extra bodies or rifles. Jackson also tells you all the soldiers passing through the Mojave outpost are sent straight to the dam or to McCarran before heading to the dam. Just about every NCR officer you talk to will tell you some version of how undermanned they are but we know the NCR is constantly trickling men into the Mojave. Oliver's strategy is what's killing the NCR in New Vegas. He hoards all the manpower he gets and has one hand wrapped around Hanlon and rangers to keep them from embarrassing him again. Left to his own devices he'll kill the NCR army as he stares down the Legion and ignores all the minor players taking bites out of his back. The Legion only stares back and laughs letting the NCR kill themselves and poke them when they can. The NCR winning the battle when it comes is less likely the longer the wait is.
However should all the chips fall into place and the battle results for a victory for the NCR I think a turnaround will be in order. That massive build up at the dam will no longer be required and the lion's share of troopers coming into the Mojave will be sent to the outposts in greater need. People like Jackson and Hayes will get that extra squad and rifles they've been praying for. The rangers will finally be no longer bound by Oliver's insecurities and free to start pacifying the frontier in earnest. All the tough hombres from Baja that were manning posts in places like Forlorn Hope are free to start spiting napalm at the threats around them. Oliver himself will be out from command to focus on his political ambitions and Colonel "kill them all" Moore will be in charge and won't be as willing to overlook threats like convict gangs shooting up whatever they please.
The Legion itself will be in a tenuous position. Caesar is on a limited time limit and Lanius either died in combat or was talked down and forced to return in failure to Caesar. Unless Caesar is playing favorites he'll make an example of him as he did the last time a legate failed him. I don't think Caesar will be around long enough especially with all the stresses this defeat will bring him to rebuild his losses, subjugate more tribes to get stronger, and come back for round 3. I don't know if Vulpes and Lucius will be characters capable of ultimately of rebuilding, strengthening and leading the Legion to continued glory. I'm also of the belief that the NCR victory in the second battle was more complete then the first time given the way the battle happens.
Most of the factions that were working for the Legion or just opposed to the NCR do make their final plays in the battle. If the NCR isn't routed at the dam they tend to hold off the threats like the fiends and cripple them in the process. Most of the groups picking on the NCR are only doing it because they feel confident they can get away with it because the Legion is camped right there drawing away their attention away from themselves. I don't think most of the minor players like the Khans or Kings won't feel like starting shit when they have a company or even a possible battalion looking right at them.
I don't think the pacification of the Mojave will be quick and decisive with the NCR depleted from the battle as they were but I do think the victory that is powerful enough to keep Kimball and possibly get Oliver into office will also be enough to keep public opinion on the Mojave annexation placated long enough for it to actually happen. It was a severe case of tunnel vision that led to the stagnation and decay of the NCR in the Mojave with the first battle of the dam being the entrance and the second battle its exit. Once the NCR is free to look around and see everything else shit will get done.
Now whether or not you want to discuss if it was a fluke that the NCR wins at I'm willing to believe it is and ultimately it will set a dangerous precedent by justifying Kimball and Oliver's actions and possibly do more harm to the NCR then the prosperity of having Vegas will do good for them in the long run.
The problem is, if the NCR wins at Hoover Dam, Kimball and Oliver will believe their strategies are working and will double down on it. Especially since the Legion is still out there, and whether or not Caesar or Lanius die, if the Courier didn't go the extra mile of eradicating the Legion high command at the Fort, there would still be Vulpes Inculta and Lucius, both of whom are far more sensible than Lanius. So if Lanius died fighting the Courier in Hoover Dam, those two will take command of the Legion, and they'll conduct the same hit-and-run tactics that have devastated NCR forces east of the Mojave. They'll coordinate strikes with the Fiends and Powder Gangers, and what's left of the Khans and the Brotherhood of Steel in the Mojave would also likely join in, provided that the Courier went for the "destroy" option against both of them, which is the easiest option.
Peace would end up being just as bloody as war as these groups bleed the NCR's numbers while the Legion masses for another attack at Hoover Dam to "avenge" the deaths of Lanius and Caesar. And since the Legion is a dictatorship, they can do that, and simply execute anyone who protests. Whereas the NCR, being a democracy, can't just keep sending bodies because the top brass told them to-they're a democracy, which means that since the people of the NCR are getting sick to death of the Mojave campaign, the politicians will have to listen when the people make their displeasure known. Oliver and Kimball can try pushing for things as much as they can, but unless the Courier does everything for them and sets the stage for a total NCR victory in the Mojave by doing all the sidequests and ensuring that the Enclave, Boomers, and Brotherhood join the NCR, NCR ownership of the Mojave will just have the same repeated tragedy as what happened after their first victory against Joshua Graham.
I have a question, what does everyone think about the fate of a post-New Vegas Legion? Let's assume that the Courier sides with anyone but the Legion. I am also assuming Caesar dies from a seizure at some point during or in the immediate aftermath of the second battle of Hoover Dam. Would Lanius have been able to be talked down or would he have been killed? Who would take charge of the Legion? Lucius, head of the Praetorian Guard, Vulpes, some other Legate? I had a suspicion that Caesar had an Octavian up his sleeve to take over in the event that he dies. Also because there is no way that Caesar did not take advantage of any of the tribal women he conquered. No cult of personality has its figurehead abstain from fucking their followers and especially not one with the arrogance of Caesar.
Depends on them winning or not.
Caesar dying is a given, unless the Courier heals him. So either Lanius takes over, or Vulpes Inculta and Lucius take over. If Lanius takes over, and the Legion lost at Hoover Dam, he withdraws from the Mojave entirely, because to him, occupying the Mojave is a logistical nightmare, and he'd probably focus on building up more strength in the east before expanding westward ever again. If Lanius dies, then Vulpes and Lucius take over as some kind of Roman-esque two-consul system, and they use subterfuge and other tactics to weaken NCR holdings in the Mojave before massing for another attack.
If the Courier helped them win at Hoover Dam, then it's more likely that the Courier will be the next Caesar. All the steps are there: the Courier was educated by Caesar on how he created the Legion and how its inner workings function. The Courier killed the NCR president Aaron Kimball and defeated his most trusted general, Lee Oliver. The Courier also killed Mr. House and eradicated the Brotherhood of Steel in the Mojave. Their deeds would be well-known by the Legion troops, and even Lanius would have to respect a person who single-handedly eradicated an entire platoon of power-armored paladins with powerful energy weapons. Plus, Caesar put the Courier's face on the money. It's the old Roman way of signaling that the Courier is next in line to inherit the throne once he passes away. The Courier, who broke the back of the NCR, eradicated the Brotherhood of Steel, killed the mysterious Mr. House, saved Caesar from his brain tumor, and led the Legion forces to victory at Hoover Dam, avenging the loss suffered by Joshua Graham and the Legion five years before. That's one hell of a resume, which basically means that the Legion ending is just a more brutal Yes Man ending where instead of Securitrons, your goons will be Lanius, Vulpes, Lucius, and the Legion as a whole, once Caesar passes away.