I think it’s definitely a Brotherhood clean sweep. The Brotherhood crushed the mutants, Beastlords, Reavers, and the Calculator’s forces, most of whom were more technologically advanced than the Legion. The Brotherhood, like the Legion, assimilates what’s left of its foes but operates at a much higher technological level than the Legion. Brotherhood squads would wipe out smaller NCR outposts while mechanized forces and power armor troops would assault major NCR bases and I personally don’t think the NCR stands a chance.

As much as I like the NCR. The Midwest brotherhood right after the calculator would have access to an entire Armada of tanks, nukes, giant mechs, vertibirds, military Humvees, a shit ton of impenetrable bases, an army of robots, access to vast swaths of advanced pre war technology. Miracle medicine in the form of bio med gel. Airships. AFVs, APCs, scouting dune buggies. Kind of a fucking lore hole that they haven't been seen or mentioned asides a little in passing in 3, NV and 4. They would be the most well armed faction in the entire United States and Could probably win a multi front war with the Enclave, NCR and east Coast and west coast brotherhood and the legion