If they were to land close to Rome, they would probably capture it before any legions could respond. If they failed to capture Rome before running out of ammo,

If they were to land close to Rome, they would probably capture it before any legions could respond. If they failed to capture Rome before running out of ammo, they would probably lose. That said, they would certainly have huge advantages, including psychological ones (superstitious Romans might mistake Marines for armies of an angry Jove). If they succeed, and set themselves up as Praetoria Novae, some of the smarter Marines might be able to accelerate Rome's military technology a few hundred years by introducing gunpowder and bronze cannons.