"Terrorism did not work against the Romans or Peter The Great or Gengis Kahn."
It seems that what you are saying is that terrorism does not work against terrorists!
Not at all. They were not terrorists. You can't judge past societies by today's standards. Rome was a very stable society that brought civilization to, at the time, unheard of heights. We still benefit today from what began in Rome.
Peter the Great brought Russia from the it's own dark ages lasting 200 years longer than the Dark Ages of Europe to a new era of enlightenment and progress.
Gengis Kahn conquered all of Asia and into Eastern Europe. Yes he was murderous to those who resisted but as with Rome his rule over conquered nations was not all that burdensome and many nations prospered, including Moscovy (Russia) while paying taxes to The kahns. Even he was no terrorist. Building empire was just what great leaders did in his day.
My point is that the societies under these seemingly barbaric leaders and civilizations performed the first duty of government more steadfastly than we do today. They protected their citezenry and kept order buy crushing their enemies.