Between 218 B.C and 203 B.C, Hannibal, the great Carthaginian conquerer, and his army, defeated in a series of battles several Roman armies, many time their match.
This series of glorious victories, which resulted with Hannibal going down in History as a tactical genius, annihilated the top generals of Rome, and on a few occasions left nothing between the invading Carthaginian army and the great city of Rome, but the strong battlements of the latter .
Thus was created the famous saying: Hannibal ad Portas! - from Latin, meaning: Hannibal is at the city-gates!
Despite the vulnerability of Rome, Hannibal made no attempt to assail it, and preferred instead citing a revolt of the Italian country-side, allowing the Romans to recover. For a few more years his army roamed and ravaged Italy, repeatedly defeating any force the Romans sent against it. But his force was dwindling, and on 203 B.C he was driven off to Africa to protect his home city, Carthage.
Eventually, Hannibal never attacked Rome, but his name remained a dread in the hearts of the Roman people, and the saying, Hannibal ad Portas, became an omen of the conquest of Rome by an invading army.
Thanks to the great system of Public Education, the first I've heard of this saying and its origins, was in the movie The Postman. Yes, the United States Postal service, three hours long, after school special, starring Kevin Costner. Where General Bethlehem, Will Patton, tells how the Romans used to scare their children, as a bed time story, by telling them Hannibal ad Portas!.