The split of the Roman Empire was a long time coming, and divisions had occurred prior to the final, permanent East-West divide in the fourth century, according to Humphries.

The eastern Roman empire was founded by Constantine the Great on May 11, A.D. 330. The empire fell on May 29, 1453, when the army of the Ottoman sultan Mehmet II breached the walls of Constantinople.

Mr. Rapley is the author, with Peter Heather, of “Why Empires Fall: Rome, America and the Future of the West.” America likes to think of itself in garlanded terms. The shining city on a hill ...