Acceptable casualties? Well after the ridiculous slaughter at the Purifier I can't imagine that the Enclave would have held back, they still had theire fleet of Vertibird's and, I'm sorry, trying to shoot-down a speeding, low-flying, aircraft with an unguided weapon not designed for anti-air defence (either the missile launcher or the tesla cannon which was based on pre-war plans and is hardly anti-air specific) is pretty [censored] impossible - especially from inside the courtyard where you'll only be able to see the birds as they go over head.
Pound the place with bombs and even the afore-mentioned artillery before sending in the boys, I can't imagine that the Brotherhood could hold on for long after that; hell just drop some Deathclaws into the place to do the work for them.
I thought OP meant before all that...
But, after defeat at PP Enclave then is not in position to attack.
RR is destroyed. LP can shoot down verts. The only time the Enclave had a window to take the Citadel with best chance of success and minimal casualties was prior to that. Was it a military blunder on the part of the Enclave, or were they just not ready?
Who knows, but military blunders happen... this is no where near the huh wha? scenarios created by NV and how both PA tech factions defeated by dweebs.
Helios and Navarro ridiculous. Btw, I recently looked at Helios. It is not surrounded by mountains. There is one mountain range... black mountain, and I highly doubt NCR was fighting from mutant territory. Sniper positions against Helios would svck.
But like I said, the militaries in FO games have been pretty weak. Except Tactics, which actually does some smart moves, taking out production facilities, etc.