Title says it all. I was thinking early about the landscape of the Fallout world, and how much is left as far as powerful factions and territory to be explored.
Be warned for minor spoilers for FNV and Fallout 4, though I will stay away from too much detail in either.
Considering that, throughout the entire of North America, with the loss of Caeser's Legion, The Enclave, the Institute and Super Mutants, the only two major "superpowers" left in post apocalyptic America are the NCR and the East Coast Brotherhood.
With the probable canon endings for both of the last two Fallout games likely placing the Brotherhood and the NCR as victors in their individual conflicts, their expansion across the Wasteland and inevitable meeting seems highly likely. There are probably some other factions out there with major power, though the most prominent are now gone, and there's not all too much of America left unexplored with the Legion having now cut a swath through the the unexplored cetral states and parts of the mid-west and wiping out a major amount of it's power before likely dissolving into squabling tribes after their loss at Hoover Dam. Without any concentrated forces inbetween, both of these Factions are free to expand East and West respectively until they meet somewhere in the middle.
My question is as to whether a second war is inevitable. Considering this is still the Brotherhood and the NCR still view them as enemies, while the new Brotherhood also desires technological control over the wasteland, their goals also conflicting in regards to wasteland dominance, what would the possibiliy or even result of such a conflict be? The last war was lost by the Brotherhood due to it's xenophobia and reluctance to recruit outsiders, an issue that does not exist in this new Brotherhood. Add to that the fact that numerous Brotherhood forces remain hidden throughout NCR territory, and would likely emerge to disrupt NCR operations in the event of such a conflict. On the other hand, the brotherhood is still unpopular in the West and they are also isolationist, with a lack of ability to communicate or organise troops, and schisms within their own order, particularly east and west are still a contentious factor. NCR possess a far larger army, but could possibly face a battle on multiple fronts against an organised, well trained and well equipped foe.
Who wins in such an event, or could a war be averted? Is this the inevitable plot of a future title?

Probably not. The NCR and the BoS wouldn't touch each other with a ten foot pole. They would probably be too scared of each other to start a new war. Maxson or whoever is Elder would shudder at the thought of becoming what happened to the Western BoS and the NCR would shudder at being obliterated by the BoS.

All in all, there'd be no war as both sides would have to be completely insane to start it. The NCR and the Brotherhood have too much to lose. If it did happen, the NCR would win as they weren't destroyed by any protagonist so far (and we know how OP the protag always is). The Courier got the closest but the NCR just retreated. The Brotherhood can be destroyed by both the Lone Wanderer and the Sole Survivor pretty easily. In Fallout 3 all you have to do is target the payload to the Citadel and in Fallout 4 you just have to reprogram Liberty Prime to take down the Prydwen. EZ.

It seems you are viewing the Brotherhood as one. While technically they are one huge faction they are split into chapters that vary in size that are controlled by a Elder. For example the Mojave Chapter would have no say in what the East Coast Chapter could do and vice versa. Though it is possible and likely that a war between the BOS and NCR is a possibility it would most likely be in the NCR's favor in terms of man power and supplies as in order to encounter the East Coast BOS the NCR would of had to come pretty far just for the East Coast BOS to hear of them.

Also the Midwestern BOS is not really lore, as Fallout Tactics is not really considered for the most part lore. Also you could argue that there are references that they are considered lore but there is no hard and compelling evidence that proves that the Midwestern BOS is actually real. Also as you said with these chapter communicating it would seem very rare and difficult as the Mojave chapter would be so far behind enemy lines and communications could be tracked and listened on.

Your theory of the NCR facing the BOS on multiple fronts would be possible on a small scale but not on a large continental scale as the East Coast BOS while being powerful is also somewhat small. Though this may have changed in the future since it would of taken such a long time for the NCR to regain strength and colonize the rest of the continent.

If it did happen, it would probably end up like pre war America did, they two factions would Nuke each other, history is bound to repeat..

@maxman885 If the war with the legion and the NCR did occur it would not result in a Nuclear Exchange between both parties. If you think this would happen please provide some evidence to prove this as an actual possibility. The NCR does not posses Nuclear weapons and even it it did it would be so small. Even going further is the NCR was to somehow launch a ICBM or fly some sort of Nuclear Bomb to a legion controlled area the damage would only effect one part of legion controlled territory. Another note would be that that the majority of the American Nuclear arsenal is depleted or in complete disarray. You could argue that "Well the Divide's Silos are still in operating condition",the only reason Ashton and Ulysses Bunker/Silo was in working condition is because of the bots keeping it alive, though this may be true for other locations. Also The Divide is unique in location because of its storms which change your body and literally remove all of your skin. This could of been the fate that the Soldiers stationed there faced. Also to note the cracks in the Earth occurred later during an NCR and Legion War.

I honestly think air power will have a huge role. As of Fallout 4 the East Coast BOS can manufacture their own vertibirds. In NV the NCR has 1 vertibird, and you could possibly consider the Boomer's aligned with the NCR, so they could have a B-29. So thats two, at the most, compared to the BOS that can build them. On the other hand the NCR has 800,000 people, and they do have their own industry back in California. It could be possible the NCR gains vertibird manufacturing, since they did wipe the enclave out of the west and have all their resources to work with now. I think a war will happen. The real question is will you choose the NCR or the Brotherhood?

I think that if have a war the brotherhood will prevail, because of the power of the NCR is very much highter than just a couple of paladins

Corvo wrote:

Probably not. The NCR and the BoS wouldn't touch each other with a ten foot pole. They would probably be too scared of each other to start a new war. Maxson or whoever is Elder would shudder at the thought of becoming what happened to the Western BoS and the NCR would shudder at being obliterated by the BoS.

All in all, there'd be no war as both sides would have to be completely insane to start it. The NCR and the Brotherhood have too much to lose. If it did happen, the NCR would win as they weren't destroyed by any protagonist so far (and we know how OP the protag always is). The Courier got the closest but the NCR just retreated. The Brotherhood can be destroyed by both the Lone Wanderer and the Sole Survivor pretty easily. In Fallout 3 all you have to do is target the payload to the Citadel and in Fallout 4 you just have to reprogram Liberty Prime to take down the Prydwen. EZ.

That didn't stop the war the first time. All it would take is the correct conflict of interests to tip the two into a war situation. It's arguable the first war was entirely started because the Brotherhood believed they were too powerful to lose, but in a similar sense, the NCR could easily inderestimate the new Brotherhood, perhaps mistaking them for just a single rogue chapter? As for the completely insane part, I agree. But then again, nuclear war is also completely insane.
It just seems like too much of a variable to have these two massive factions in the wasteland, particularly with a history of violence between them.
As for the ability to destroy them, in Fallout 1 you can destroy the NCR in it's infancy by wiping out Shady Sands, so I wouldn't say the NCR is unbreakable. The NCR in Fallout New Vegas also hinges on economic disaster and possible invasion should the Legion win, though they are certainly still alive in either result, though so are the Brotherhood in Fallout 4/3 even with the Prydwen lost or Citadel destroyed. You're certainly doing damage to them, but they still exist.

I think it's important to note that the West Coast BOS and the East Coast BOS are very different from one another. While the West Coast have become more isolationist and more solely focused on retrieving technology and remaining concerned with nobody but themselves, the East Coast has taken a very different route. if we are talking in terms of the current East Coast run by Maxson, then they have reverted a little bit to the original ways of the BOS, where they are less accepting, if at all, of anyone but humans (as a side note to this: while we don't really see much of it in Fallout 3, Lyons' BOS was still against ghouls and mutants in any regard, it was just not as touched upon as it was in Fallout 4). The East Coast BOS is much more concerned with actually helping the wasteland get back on its feet and become a successful region not threatened so much by super mutants and raiders. They have a genuine desire to help people.

With all of that in mind and knowing what we do about the NCR, if the East Coast BOS and NCR were to cross paths, i highly doubt there would be a full out war. Perhaps there would be some general distrust of one another with some squabbles here and there, but nothing major. I would also like to point out that the odds of the East Coast BOS moving all the way towards the Western part of the U.S. Wasteland is highly unlikely, especially considering the fact that the West Coast BOS still exists in some degree. The ones in New Vegas that were killed off as a part of the NCR ending were not the only ones in the region. There is also the consideration that securing the rest of the East Coast region would take many years. So if they were to meet, it would take many many years and would more than likely end in nothing more than an agreement to stay out of each others' way.