"Before their empire fell, the Romans built walls. They began by erecting barriers along the border following the death of the Emperor Trajan in 117 A.D., notably Hadrian's Wall, which belted Britain. Later emperors erected internal walls, even around the great city itself, to ward off barbarians. After 300 A.D., the Emperor Diocletian effectively converted the entire Roman populace into feudal serfs, walling them off from internal movement in a vain effort to stabilize the chaotic economy.
"Despite the cautionary tale of Rome, building walls, both literal and figurative, has remained a habit of great powers in decline -- the fateful course taken not only by Ming China, but also Soviet Russia, and even Great Britain. Sadly, many Americans are all too eager to repeat history."