"tu regere imperio populos, Romane, memento;
hae tibi erunt artes, pacique imponere morem,
parcere subiectis, et debellare superbos."

(Remember, Roman, that these are your arts: to rule the peoples of the earth by your power, to impose law on top of the peace that ensues, to spare those who subject themselves to you, and to wage ceaseless war upon the proud.)

That’s what it means to be an imperial power. It’s what Alexander the Great did when he marched his army through Mesopotamia in the late fourth century B.C. Alexander didn’t bother with cultural diversity; he set up Greek city-states colonized and run by his own soldiers wherever he marched.
The Romans did the same thing. As Virgil wrote in the Aeneid, it was the Romans’ job to rule. The Romans conquered the world because they had a vision of their own mission to the world, which was to make it safe for trade, the rule of law, and the flourishing of classical civilization.
This was a vision that the Romans knew had to be imposed by force, which the Romans knew how to use - and weren’t afraid to use - because they had the best army in the ancient world.
They punished violations of the Pax Romana ruthlessly. Touch so much as a hair on the head of a Roman citizen, and you got yourself and your pals promptly crucified and your town razed to the ground. The Romans had limitless self-confidence because they believed that they had a mission to civilize.
Same went for the Spaniards when they set up their New World empire 1,500 years later. Same for the French and the British during the 19th century.

The Americans, sad to say, seem to believe that we can somehow create a world free of terroristic threats to our citizens and our allies and still be nice. That we can rule the world by persuasion, by talking people into wanting to be just like us. This is a mistake. This is the Fallujah mistake.

And over time, if we keep it up, Americans, like the Romans in their last years in the West making concession after concession to the barbarians, will pay for that mistake with dear coin.

Report note: my opinion is different. Perhaps the author did'nt read the 'De Bello Gallico' by Julius Caesar, and moreover we've passed trough the christianism. Nothwithstanding this, it's a quite interesting opinion, caught it up from the cyberspace.
The truth is that the peoples who came into contact with the Romans recognized their cultural superiority and assimilated works, customs and law often without need to clash.

Barbarian assaults and fell of the Western Empire