Hannibal ad portas! The phrase was enough to terrify anyone in the Roman Republic and became an adage for parents to scare their children at nights: “Hannibal is at the gates.” The Carthaginian commander nearly destroyed Rome in the 3rd century BC and posed the republic's greatest threat until the empire collapsed in the fifth century AD. What made the general such a formidable foe? His genius for military strategy, willingness to use any level of violence necessary (Hannibal's religion called for child sacrifice), and clever use of resources.