The marines would have to use their superior firepower to enlist recruits, maybe inspire major defections from the Roman government and military. They need to leverage the awe into some kind of capitulation by the Romans.
But if they just start out somewhere and commence fighting, they will run out of ammo and fuel.
The point about Hannibal is that the Romans [of that period] didn’t surrender. The Senate refused to ransom captives from Cannae. Survivors were banished to Sicily [despite Rome’s need for trained manpower. The Senate would not allow captives’ families to ransom them. The romans refused to receive Hannibals peace emissary after the battle of Cannae. They never listened to his peace terms. When Hannibal finally got to Rome, the Senate sold the land he was camped on at full market price. Romans didn’t surrender. they just doubled down.
Goes the other way.
IF there isn’t a national command authority, the Romans might be able to conquer the Marines with a few mule loads of gold, a train of cute female slaves, and an offer of Roman citizenship.