The Roman historian Sallust understood the decline of the Republic to begin in 146 BC, when the city of Carthage was destroyed.
The line Carthago delenda est, a rallying cry of Cato the Elder against their North African city-state rival, is well known today. More obscure is an argument made at the time (at least per Sallust) against destroying Carthage: that the city provided a sense of metus hostilis (fear of the enemy) which kept Romans from turning on each other. Carthage was Rome's enemy for a total of 118 years, during most of which there was an uneasy peace. Sallust contrasted the civil wars and political usurpers of his own time with the relative sense of unity and patriotism which prevailed in that century.
So what do you think? Is this concept applicable to our time and place?