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[Fallout] What factors have allowed large powers to rise to prominence in the West such as the NCR and Caesar's Legion, and what factors have prevented similarly large powers rising in the East?

So in the 210 years since the Great War, seemingly only two powers have risen to great prominence and amassed a large swath of territory, the NCR, and Caesar's Legion, and both of them are based in States in the West, namely California, and Arizona, as well as the surrounding states that each power controls. 

As of 2281, the NCR has roughly 700,000 citizens, a sophisticated infrastructure system across its territories, a functioning manufacturing base and resource extraction industry, a thriving economy, a large, well organised, relatively well equipped, professional volunteer military, capable of overpowering the Brotherhood of Steel, the uncontested most technologically advanced force in the wastelands (since the fall of the Enclave on the West Coast), as well as their own small Vertibird fleet, and extremely well trained special forces such as the NCR Rangers and the First Recon Snipers (renowned for singlehandedly preventing Caesar's Legion from overwhelming NCR Forces at the First Battle of Hoover Dam).

Caesar's Legion itself is also quite formidable, having conquered something like 86 tribes and now in control of the territories of those tribes, and the Legion is remained uncontested until it met the NCR at the Mojave Wasteland. They're so uncontested that caravans and travellers prefer going through Legion territory because it's safer, as the Legion has practically wiped out any semblance of raiders and mutant abominations, something the NCR hasn't yet achieved. They also have a massive slave force feeding the Empire and the Legion war machine. Their ranks are filled with extremely well trained and experienced legionnaires, and they have arguably the single most effective espionage units in the wastelands, capable of infiltration, intelligence gathering, and sabotage deep into enemy territory, a capability that the NCR seems to be lacking. They're also not averse to simply throwing bodies at a problem like cannon fodder to overwhelm the enemy, and they can keep doing it because of how dedicated Legion soldiers are, and because they keep feeding soldiers to its war machines by taking the children of slaves.

These are both sophisticated, well organised, well supplied powers, rising in the ashes of a nuclear wasteland. If they were capable of achieving this in just 200 years since the Great War, why haven't any similarly large and organised powers risen on the East Coast? What is preventing this from happening?

So far we've visited the Capital Wasteland and the Commonwealth on the East Coast. The Capital Wasteland is basically still a hellscape, relative to the other wastelands we've visited. And that's quite understandable as it was hit harder than pretty much any other part of the United States during the Great War as it was the seat of the US Government. So I can understand why a large regional power hasn't risen from the Capital Wasteland. But the Commonwealth is different. Even with the threat of The Institute and their synths, and the arrival of the Brotherhood of Steel, there seems to be a large number of settlements, thriving trade between them, a much smaller and manageable Super Mutant presence compared to the Capital Wasteland, and even a volunteer police force of sorts in the form of the Minutemen. If you talk to the settlers, one thing they all have in common is the desire for security from raiders and mutants, and the ability to feed themselves, both of which are something that can be provided for by a regional power, like the NCR does in its own territory. Now at one point the Commonwealth did get close with the Commonwealth Provisional Government. Even The Institute was seemingly a part of this arrangement, sending their own representative, but something changed and the Institute's synth representative gunned down every representative of each settlement at the formal meeting to decide the governing constitution of the CPG. Now Nick Valentine says they never tried again because nobody trusted each other anymore after that massacre, but that doesn't really make sense to me because everyone seems to know that it was the Institute that massacred the representatives, so they have no reason to not trust each other. Of course the collapse of the Minutemen didn't help, but that only seemed to have happened in the months leading up to the Sole Survivor getting out of Vault 111, as opposed to the massacre which took place decades before, so why hasn't a regional power grown from the Commonwealth?

Aside from the Capital Wasteland and the Commonwealth, there must be other places along the East Coast that are in various stated of stability, so why hasn't a large regional power that people in the Capital Wasteland and Commonwealth are aware of risen yet? What is preventing something like the NCR or the Legion from growing in the East Coast?

TL;DR The West Coast has produced two large regional powers in the form of the NCR and Caesar's Legion, so why hasn't the East Coast been able to do the same?
gpt
Lots of things:

- The east coast was hit *very* hard in the war, much harder than the west coast, to the point where even the water was radioactive two hundred years after the war.  *That is not even how radioactivity normally works.*  on the west coast, we only have one target (the Glow) that was hit that hard, and it was a top secret military based isolated from all the other population centers.  In Boston, we have the glowing sea right at the edge of the city.  We also have the entrance to vault 87 just a few miles from downtown DC, which is so radioactive even a full suit of power armor and full dose of RAD-X wont let you live more than a few minutes.

- On the west coast, the Enclave was actively assisting at least one group (the Salvatore family), and mostly ignoring everyone else, since they assumed those groups would be no threat to the Enclave, and everyone would just get wiped out by the modified FEV anyways.  On the East coast, the Institute was actively infiltrating groups of wastelanders and sabotaging their efforts at getting organized.  

- The west coast had the Brotherhood of Steel helping out in the very early stages of the NCR.  Back in their early days the Brotherhood would still help clearing out raiders and trading with Caravans.  This dramatically reduced the number of threats to civilization that fledging states like the NCR needed to deal with, and allowed for regional trade routes to get set up much more quickly than in the east.

- Lastly, Caeser's legion is something of an anomaly.  It is a very large state, but it is not a stable one, and it depends on the cult of personality developed by one exceptional individual.  It grew very quickly, but is expected to fall just as quickly when he dies.