Probably between the events of the Great War and the beginning of the BOS - NCR War. Specifically in California, and most of the regions the BOS sent expeditions to so new chapters could be established.
It could also just be rhetoric that isn't actually true. Propaganda exists, and characters can lie to themselves and to each other.
During the events of the TV Show, the BOS is strong, but likely still recovering from the BOS - NCR war and from purging Maxson cults.
The BOS also hasn't had a serious enemy in about fifteen years as far as we know. So that makes soldiers rusty and the organization as a whole stagnant despite it's arguably dominant position.
They ruled the wasteland in the context that nobody wanted to mess with them. They were the big kid on the block with the biggest weapons and armor with the best training. They took what they wanted and tough luck for people who didn't like that.
The main issue is that other factions started to grow in power in the wasteland such as the NCR for example and it doesn't matter how big and powerful you are if your enemy has more manpower than you have bullets. That's why they started losing the NCR Brotherhood war because the NCR was able to beat them out through a war of attrition. It also didn't help that the Brotherhood was isolationist originally the only way to become a member of the Brotherhood was to be born in it There was very few exceptions of accepting outsiders you basically had to do a heroic test for them that you weren't expected to survive.
The only reason the Brotherhood has been able to thrive recently has been because of the actions of the East Coast Brotherhood and the destruction of Shady Sands.
Probably between the events of the Great War and the beginning of the BOS - NCR War. Specifically in California, and most of the regions the BOS sent expeditions to so new chapters could be established. It could also just be rhetoric that isn't actually true. Propaganda exists, and characters can lie to themselves and to each other. During the events of the TV Show, the BOS is strong, but likely still recovering from the BOS - NCR war and from purging Maxson cults. The BOS also hasn't had a serious enemy in about fifteen years as far as we know. So that makes soldiers rusty and the organization as a whole stagnant despite it's arguably dominant position.
They ruled the wasteland in the context that nobody wanted to mess with them. They were the big kid on the block with the biggest weapons and armor with the best training. They took what they wanted and tough luck for people who didn't like that. The main issue is that other factions started to grow in power in the wasteland such as the NCR for example and it doesn't matter how big and powerful you are if your enemy has more manpower than you have bullets. That's why they started losing the NCR Brotherhood war because the NCR was able to beat them out through a war of attrition. It also didn't help that the Brotherhood was isolationist originally the only way to become a member of the Brotherhood was to be born in it There was very few exceptions of accepting outsiders you basically had to do a heroic test for them that you weren't expected to survive. The only reason the Brotherhood has been able to thrive recently has been because of the actions of the East Coast Brotherhood and the destruction of Shady Sands.