1. First and foremost try to find a diplomatic solution before unleashing death, even forceful "knife to the throat" (~submit and pacify or face extirpation) diplomacy, and keep a back door open to a diplomatic solution throughout.
2. Send assassins and/or commando units after their Hagravens and leadership.
3. Offer amnesty to anyone who is willing to turn himself/herself in and pacify.
4. "Evacuate" their civilians and their civilian supporters away from the theater. (<- to deny the militant Forsworn support as much as possible)
5. Destroy and/or occupy the Forsworn supply system.
6. Place strategic attacks, driving them into controllable fortresses.
7. Lay siege to those places.
8. http://en.wiktionary.org/wiki/exstirpo them.
Off course the legion would rely heavily on the dragonborn hero/leader to be victorious. I doubt it'd be able to succeed without his/her prowess.
It did work for the Romans. Rome fell because of a series of events, but rebellion by subjected peoples wasn't really a factor in it's fall, and the massive germanic invasions that ultimately dismantled the western empire were a symptom of an empire that was already ready to fall, not the underlying cause of it.
Rome manly fell due to four factors: it's failed economic system, continuous exterior threat, continuous internal Roman struggle for power, and as it aged it's often paralyzing leadership / internal politics. Also, empires can only expand as much as their area of comfort, their communication lines and their sustainable supply abilities allow. Which means there will always remain enemies, since once an Empire reaches it's maximum ability to expand, those enemies will be more or less out of reach. The Romans reached their limits in the forests across the Rhine and Danube, Mesopotamia and Armenia.
The Roman interior within their borders was relatively peaceful and safe (few exceptions) until the empire had already set on a continuous path of internal degradation.
For it's conquests Rome relied heavily on brute force and "knife to the throat" diplomacy to force subjection or assimilation. Those who wouldn't cooperate were extirpated. It did work for them. Rome and it's empire peaked as a result of the resulting Pax Romana. The Romans themselves broke the Pax Romana, embarking on a destructive series of internal civil wars and struggles for power. It ruined their already flimsy economic system, and readied the empire for dismantling.


