The Roman Empire arose as a consequence of invasions of Italy, first by the Gauls who burned the city, then by the imperialists of Carthage (Hannibal spent fifteen years in Italy, able to win a couple of famous battles, but unable to take any walled cities).
The Roman army became a standing one, and imposing Roman authority on troublesome neighbors became a sustaining pattern. The empire wouldn't have tottered on for 1800 years (from the 500 BC conquest of Ostia to the fall of Constantinople 39 years before Columbus set sail) if they'd been in a constant state of war.
Their neighbors liked having the local Roman order nextdoor and as trading partners. New drifts of violent a-holes from the east went on intermittently not only until the fall of the western half of the Empire, but for centuries more, helping to produce the map of Europe today.
And the a-hole from the East is still intent on conquering as far to the West as he can get away with. More change in the process of staying the same. If only some of the blind people here will open their eyes and see.