Let's say the Legion pushes the NCR out of the Mojave and the NCR is at a minor setback. The Legion advances into New California but is stopped by the Brotherhood of Steel. Let's assume the NCR-Brotherhood War has ended. The Legion begins recruiting and has hundreds of fresh new recruits. Who would win? The Brotherhood of Steel or Caesar's Legion?
In my opinion, if the East Coast Brotherhood, a small not as powerful part of the West Coast Brotherhood managed to beat the shit out of the Enclave, then the California Brotherhood can easily send the Legion to Hell. The average Legion soldier is armed with leather armor and a machete. The average Brotherhood initiate has combat armor and combat rifles and lasers. So, yeah. But the Legion makes up with tons of soldiers and slaves. The California Brotherhood is strict on who joins the Brotherhood and therefore only has limited amounts of troops.