Ammianus Marcellinus concludes his History, the Res Gestae, thus:
These events, from the principate of the emperor Nerva  to the death of Valens , I, a former soldier and a Greek, have set forth to the measure of my ability, without ever (I believe) consciously venturing to debase through silence or through falsehood a work whose aim was the truth. The rest may be written by abler men, who are in the prime of life and learning. But if they chose to undertake such a task, I advise them to forge their tongues to the loftier style.Res Gestae XXXI, 16
From other remarks within the work, it can be deduced that Ammianus was born around the year 330 and was probably a native of Antioch, Syria (now Antakya, Turkey); that he was well connected – he was probably the son of a high ranking officer in the Roman army; that his own military career did not turn out as successfully as it might; and that he eventually settled in Rome, where, probably between late-389 and mid-391, he published the Res Gestae.
Ammianus was Greek but he wrote in Latin. The Res Gestae – covering the period from 96 (picking up from where the last major Latin history, by Tacitus, left off) to 378 – comprised thirty-one books, but only books fourteen to thirty-one survive.[*] Perhaps surprisingly, the extant eighteen books – over half the original work – just deal with the few years from 353 to 378.
The surviving books of the Res Gestae record times that Ammianus lived through, and, most vividly, events he was involved in.[*] Ammianus’ literary style is, though, rather bizarre. In the Introduction to his translation, in three volumes (Loeb, 1935–1939), John C. Rolfe observes that “some of his peculiarities are an unnatural word-order, attempted picturesque and poetic forms of expression, and a general striving for effect”, whilst in the Preface to his selective, single volume, translation (Penguin, 1986), Walter Hamilton comments: “any attempt at a close rendering in modern English would be unreadable”. Edward Gibbon, in various footnotes to Chapter 26 of his influential masterwork, The History of the Decline and Fall of the Roman Empire (six volumes, 1776–1788), voices his complaints: “Such is the bad taste of Ammianus that it is not easy to distinguish his facts from his metaphors”; “superfluous prolixity is disagreeably balanced by his unseasonable brevity”; “those turgid metaphors, those false ornaments, that perpetually disfigure the style of Ammianus”; “the vices of his style, the disorder and perplexity of his narrative”. Despite his criticisms, however, Gibbon greatly admired Ammianus – “this impartial historian”; “the impartial Ammianus” – and, famously, bids him a fond farewell:
It is not without the most sincere regret that I must now take leave of an accurate and faithful guide, who has composed the history of his own times without indulging the prejudices and passions which usually affect the mind of a contemporary.Decline and Fall Chapter 26
Modern historians, however, tend to be considerably more sceptical than Gibbon seems to have been about Ammianus’ self-professed reliability and impartiality.
Ammianus was a pagan. Julian, who was the last pagan emperor, is the undoubted hero of the piece – his reign (as Caesar 355–360, as Augustus 360–363) spans eleven books (XV–XXV) of the Res Gestae. Ammianus admits (XVI, 1) that his account of Julian’s deeds “will almost belong to the domain of the panegyric”, but insists that “no wordy deceit adorns my tale, but untrammelled faithfulness to fact, based upon clear proofs, composes it”. He does have some criticisms of Julian. For instance:
… [Julian was] too much given to the consideration of omens and portents, so that in this respect he seemed to equal the emperor Hadrian. Superstitious rather than truly religious, he sacrificed innumerable victims without regard to cost … the laws which he enacted were not oppressive, but stated exactly what was to be done or left undone, with a few exceptions. For example, it was a harsh law that forbade Christian rhetoricians and grammarians to teach, unless they consented to worship the pagan deities.Res Gestae XXV, 4
Ammianus gives the impression of being tolerant of Christianity – a “plain and simple religion” (XXI, 16) – but whether this is just an impression or not – whether he was actually anti-Christian – is the subject of ongoing debate. Nevertheless, Ammianus Marcellinus is generally touted as ‘the last great historian of Rome’. Averil Cameron sums up:
He is a major historian on any count, and one whose range and geographical scope are far greater than had previously been seen among Roman historians. He belongs to his age in many ways, not least in the fact that he writes as a member of a bureaucracy whose officials were liable to find themselves in any part of the empire, and who were in close contact with numerous others like themselves, both civilian and military. It is Ammianus who gives us our best information both about the late Roman army in action and about the extraordinary juxtapositions of life in imperial and senatorial circles. Finally, it is Ammianus the writer who, once read, is never forgotten.The Cambridge Ancient History Volume XIII (1998), Chapter 22, §VI
Ammianus Marcellinus Res Gestae translated by John C. Rolfe
In 359, the Persians invaded Mesopotamia. Ammianus’ account of his own experiences at this time make exciting reading, as the following extract demonstrates:
… we planned to hasten to Samosata, in order to cross the river from there and break down the bridges at Zeugma and Capersana, and so (if fortune should aid us at all) repel the enemy’s attacks. But there befell a terrible disgrace, which deserves to be buried in utter silence. For about seven hundred horsemen, belonging to two squadrons who had recently been sent to the aid of Mesopotamia from Illyricum, a spiritless and cowardly lot, were keeping guard in those parts. And dreading a night attack, they withdrew to a distance from the public roads at evening, when all the paths ought to be better guarded. This was observed by the Persians, and about twenty thousand of them, under the command of Tamsapor and Nohodares, passed by the horsemen unobserved, while these were overcome with wine and sleep, and hid themselves with arms behind some high mounds near Amida.
And presently, when we were on the point of going to Samosata (as has been said) and were on our way while it was still twilight, from a high point our eyes caught the gleam of shining arms, and an excited cry was raised that the enemy were upon us; then the usual signal for summoning to battle was given and we halted in close order, thinking it prudent neither to take flight when our pursuers were already in sight, nor yet (through fear of certain death) to engage with a foe far superior in cavalry and in numbers. Finally, after it became absolutely necessary to resort to arms, while we were hesitating as to what ought to be done, some of our men ran forward rashly and were killed.…
While all this took place in the course of half an hour, our soldiers in the rear, who occupied the higher part of the hill, cry out that another force, of heavy-armed cavalry, was to be seen behind the others, and that they were approaching with all possible speed. And, as is usual in times of trouble, we were in doubt whom we should, or could, resist, and pushed onward by the weight of the vast throng, we all scattered here and there, wherever each saw the nearest way of escape; and while every one was trying to save himself from the great danger, we were mingled in scattered groups with the enemy’s skirmishers. And so, now scorning any desire for life and fighting manfully, we were driven to the banks of the Tigris, which were high and steep. From these some hurled themselves headlong, but entangled by their weapons stuck fast in the shoals of the river; others were dragged down in the eddying pools and swallowed up; some engaged the enemy and fought with varying success; others, terrified by the dense array of hostile ranks, sought to reach the nearest elevations of Mount Taurus. Among these the commander himself [Ursicinus] was recognised and surrounded by a horde of warriors, but he was saved by the speed of his horse and got away, in company with Aiadalthes, a tribune, and a single groom.
I myself, having taken a direction apart from that of my comrades, was looking around to see what to do, when Verennianus, one of the guard, came up with an arrow in his thigh; and while at the earnest request of my colleague I was trying to pull it out, finding myself surrounded on all sides by the foremost Persians, I moved ahead at breathless speed and aimed for the city [Amida, now Diyarbakır in Turkey], which from the point where we were attacked lay high up and could be approached only by a single very narrow ascent … Here, mingled with the Persians, who were rushing to the higher ground with the same effort as ourselves, we remained motionless until sunrise of the next day, so crowded together that the bodies of the slain, held upright by the throng, could nowhere find room to fall, and that in front of me a soldier with his head cut in two, and split into equal halves by a powerful sword stroke, was so pressed on all sides that he stood erect like a stump. And although showers of weapons from all kinds of artillery flew from the battlements, nevertheless the nearness of the walls saved us from that danger, and when I at last entered the city by a postern gate I found it crowded, since a throng of both sexes had flocked to it from the neighbouring countryside. For, as it chanced, it was at that very time that the annual fair was held in the suburbs, and there was a throng of country folk in addition to the foreign traders. Meanwhile there was a confusion of varied cries, some bewailing their lost kindred, others wounded to the death, many calling upon loved ones from whom they were separated and could not see because of the press.Res Gestae XVIII, 8