Ammianus probably had a literary education, if we credit his narrative’s frequent allusions to Cicero, Sallust, Herodotus, Thucydides, and Polybius. He may have coveted a distinguished military career, which went unfulfilled, possibly on account of his public association with the pagan emperor Julian the Apostate.
In Antioch during his mid-20s Ammianus entered an elite military corps, the protectores domestici (household guards), possibly as an intelligence officer. He served on the personal staff of Ursicinus, governor of Nisibis in Mesopotamia, as well as the mater militiae. Recalled to Italy with Ursicinus in the mid-350s c.e., he later accompanied his mentor to Gaul to depose the famously unfortunate Frankish imperial usurper Silvanus.
Returning with Ursicinus to the East in 359, Ammianus barely escaped a ferocious Persian attack on Amida (Diyarbakir, Turkey). Ammianus spent the next few years in Antioch until he joined the emperor Julian’s campaigns in Gaul and in the East.
After Julian’s death in battle in 363 Ammianus again repaired to Antioch. His activity for the next 20 years is mostly unknown. He traveled to the Black Sea, Egypt, Greece and, possibly, Thrace around the time of the Goth invasions there, 376–378.
Ammianus does not reappear until 383 in Rome during a food shortage in which foreigners—except some 3,000 foreign dancing girls—were forced out of the city. It is likely that Ammianus came to Rome to interview eyewitnesses of significant contemporary events and to access official records.
The first draft of Ammianus’s Res gestae (Deeds of men) begins his history in 96 c.e., on the accession of Nerva, and stops in 364, with the death of Jovian soon after Julian’s demise (Books 1–25). It appears that favorable public reception to recitations of his manuscript persuaded him to add six more (Books 26–31).
The first 13 books tracing Rome’s course from 96 to 354 are lost, and the 18 books extant cover only 25 years, from 354 to 378. This segment is a precious survival because only fragments exist of his contemporary Eunapius’s history; his fellow soldier Eutropius’s Breviarium ab urbe condita and the work of Aurelius Victor all are mere summaries that end before 378.
Following Thucydides, he reproduced important speeches and included gossipy character sketches. More to modern taste, Ammianus broke with traditions to depict contemporary social, economic, and cultural life.
Though still controversial, his usually objective treatments of Christianity—perhaps motivated by a desire to win a wide readership—are likely the most impartial perspectives on this topic of any ancient writer.
Much admired by Ammianus, Julian’s virtues are duly enumerated and lauded, but the historian also details a catalog of Julian’s defects of judgment. By comparison, neither Julian’s predecessors such as Constantius II, nor his successors, Valentinian I and Valens, fare so well in Res gestae.
Ammianus’s Res gestae is the most important work on ancient Roman history after Tacitus. It was composed by a patriotic citizen of the empire who possessed an idealized moral vision of the Roman past and was anxious about the prospects of Rome’s civilization in the future.
While his text is often florid and gossipy, it is clear that Ammianus Marcellinus—"a soldier and a Greek" (31.16.9) who wrote in Latin in Rome—tried to deliver what he promised to his readers: an account of the deeds of men that is competent, accurate, and often comparable to the best models of ancient historiography.