September 11, 2001, is frequently compared to December 7, 1941, as a day that will "live in infamy." But a more appropriate analogy might be August 24, 410, when the city of Rome was besieged and pillaged by an army of 40,000 "barbarians" led by the Osama bin Laden of late antiquity, a wily warrior named Alaric. One can still see the effects of this cataclysmic event when walking through the ruins of the Roman Forum today. The Basilica Aemilia was the Wall Street of ancient Rome, a beautiful structure in the Forum with a marble portico. One can still see the green stains of copper coins melted into the stone from the conflagrations set by Alaric and his marauders.

Before then, Roman coins bore the legend Invicta Roma Aeterna: eternal, unconquerable Rome. It had been more than 800 years since the Eternal City had fallen to an enemy's attack. In many ways, Rome was like America prior to 9/11, the world's only superpower. But in 410, Rome's military power could not prevent its walls being breached, its women raped, and its sacred precincts burned and sacked.

When Jerome heard about the fall of Rome in faraway Bethlehem, he put aside his Commentary on Ezekiel and sat stupefied in total silence for three days. "Rome was besieged," Jerome wrote to a friend. "The city to which the whole world fell has fallen. If Rome can perish, what can be safe?" The British monk Pelagius, who was in Rome when the attack occurred, gave this report: "Every household had its grief, and an all-pervading terror gripped us."

Responding to those who said Rome fell as the gods' punishment against the ascendant Christians, Augustine, the bishop of Hippo in North Africa, began writing The City of God, an opus magnum et arduum, as he called it—a "great ...