The Legion is really not sustainable. It reminds me of ISIS or Khmer Rouge - an organization based on rigid ideology and terror, trying to recreate old forms and breath new life into them.
ISIS tried to recreate a medieval caliphate, Khmer Rouge, while decraling adherence to communism, really strived to form a pre-modern society based on rural communities. Despite their militaristic nature, they both failed, defeated by stronger and more able foes.
The legion is also a militaristic organization that lives in the past, it has its strict ideology and is a shitty nation-builder. They may provide short term security on occupied territories, but those territories also stop developping under the iron rule. Legion hates modernity in all forms, including weaponry and medicine - it is described in several places of the lore. Under Ceasar there would be no research and development, no scientific breakthroughs, no culture or art (cultural diversity of tribes forming the Legion is crushed). Only constant war. And when there are no more foes - than nothing else remains.
Besides, due to their limiations, Legion's soldiers aren't even that effective in combat. Sure, they defeat primitive tribes, but when faced with a real foe, they usually shit the bed. They lost the Battle of the Hoover Dam, didn't they? And to whom? To the corrupt, decadent traders of the NCR.
That is a fair criticism but the question is that the Khmer Rouge and ISIS were trying to create a new society completely from ideology, there was no basic drive and natural cultural strong pressure in favor of that, any attempt of pushing ideology like that without a natural fertile ground, something real to back that up, will fail, the ideology of ISIS wasnt much different from the early islamic conquest but on that case, they were really successfull while on the case of ISIS, ISIS failed. Why? Because before the islamic revolution, you already had a warrior society with the right conditions, you just needed something to unify it. It wasnt Islam that created the middle age arabic culture but it was what united it and gave it new meaning. In the case of ISIS, there was no warrior culture on Syria, you cant create something from the scratch with just ideas. ISIS was wishing for something that didnt exist anymore.
On the case of Ceasar, there are already tribes, already desperate groups desperate for some order and direction and plenty of people desperate to survive as surviving was hard, the Wasteland was a hellhole that applied pressure more than enough into a direction of the strong must survive and Ceasar only needed to take something that already existed and give a new spin to it, in the end, the strongest cultures are the ones that fit the enviroment they are in. The biggest inconsistency on Fallout lore however is the Wasteland hellhole scenario vs the rise of new civilization scenario and the two scenarios are competing and lead to inconsistencies, so people might point that the Legion makes no sense because something like the NCR can exist, if something like the NCR can exist then the Legion is unnecessary while at same time you have raiders and no law pratically everywhere.
New Vegas tries to bring together the two scenarios by implying that the NCR is on the brink of collapse and they only won the first battle of the Hoover Dam out of pure luck because their ruse on Boulder City actually worked but they had to pretty much bomb themselves to win and it was Pyrrhic victory, before that, they were losing badly, if the battle was on an open field, they would had been slaughtered and they have no way to actually control the Wasteland as a Estate. About technology, that is kind of inconsistent because the Legion rejects most of what the NCR believes but they cleary want the Dam, if they were luddites, they wouldnt be having all this effort to have it.
Also, New Vegas is inconsistent with logicstics, no way a resource starved post apocalyptic Estate would have guns and ammo to outfit everyone like NCR does and actually, a mix of melee weapons and guns for a few units would be the norm not out of rejecting technology but more because you really dont have the vehicles and livestock to keep carrying bullets for miles on scorching desert to keep feeding bullets to thousands of soldiers on a acceptable time frame. I guess the designers prefered for NCR having a more distictive second world war US army style and look different from the Legion so they kinda hand waved that away but the realistic take would be more like the Legion style of combat.
are like wombyn they have no honor.
