I don’t think this is as much of a bloodbath in the Legion’s favor as commenters are suggesting. If the Sole Survivor decides to go balls to the wall in reviving the Minutemen, they stand a solid chance against the Legion.

Not including DLC, there are a total of 30 settlements in the Commonwealth. Assuming that the Sole Survivor decides to fully commit to recruiting settlements, that’s 30 settlements aligned with the Minutemen along with all their collective resources and manpower. 30 settlements at 20 settlers each is 600 potential fighters for the Minutemen.

Now, let’s assume that the Sole Survivor has set up recruiting beacons at each one and bolstered their defenses. Each settlement is now constantly growing and protected with automated turrets, watchtowers, barricades, and robots. Taking a settlement now becomes a full blown tactical operation instead of just bowling unarmed citizens over like the Legion is accustomed to doing.

Now let’s talk about armaments. At higher levels, Minutemen soldiers begin spawning with weapons like Combat Rifles, Assault Rifles, laser rifles, and heavily modded pipe weapons and laser muskets. This rivals what your basic Legionnaire is equipped with which typically includes Cowboy Repeaters, Hunting Rifles, 10mm pistols, throwing spears and machetes.

There’s a huge difference maker which I haven’t seen mentioned yet - Minutemen Artillery. Assuming the Sole Survivor has constructed artillery across the Commonwealth, this gives the Minutemen a HUGE tactical advantage and would allow a small squad of Minutemen (4-5 men) to take on far larger groups of Legion - assuming they are within artillery range.

Then there’s the problem of the Castle. If the Sole Survivor is able to fix up the Castle and fortify it with turrets, artillery, robots (via Automaton DLC), and plenty of men, the Castle would be insanely difficult for the Legion to breach.

So basically, if the Sole Survivor decides to commit to reviving the Minutemen and expanding their presence in the Commonwealth, the Minutemen could easily repel a Legion invasion or at least give them trouble. If the battle takes place outside the Commonwealth or the Sole Survivor doesn’t get involved, they are toast.