In A.D. 175, a courier presented the emperor Marcus Aurelius with a letter from the Roman Senate. The news was not good: Marcus had been betrayed by the military commander Avidius Cassius, who had been hailed as emperor by the Egyptian legion in Alexandria. Marcus had recently been in poor health, and there were rumors—possibly spread by Cassius—that he was dead or dying. Now the frail and aging emperor had to prepare for war. As Donald Robertson writes, Marcus found himself “confronting one of the most serious crises of his reign.”

Though...