One spring morning in 359, shortly after sunrise, from a mountain somewhere in the region of Corduene (in the frontier region between modern Turkey and Iraq), a Roman officer on a reconnaissance mission spotted a vast Persian army on the horizon.1 The army had been assembled by the Persian Great King Shapur II (309-72). Soon the soldiers would cross the border into Roman Mesopotamia. Shapur’s last campaign against the Roman emperor Constantius II, Constantine’s last surviving son, was another act in the bitter and enduring struggle between Romans and Sasanians for control of the Fertile Crescent.2 Thirty years after his tour of duty in Persian territory, and now a retired officer in Rome, Ammianus Marcellinus wrote his Res Gestae, a 31-book continuation of Tacitus. In books 18 through 20 he gave a detailed account of Shapur’s invasion. It is his most extensive account of a war conducted under a Christian emperor.