Reference edition:

Ammiani Marcellini Rerum gestarum libri qui supersunt, edidit W. Seyfarth, Leipzig 1978 (Bibliotheca Scriptorum Graecorum et Romanorum Teubneriana). 

The work of Ammianus Marcellinus, entitled Res gestae according to the codex Vaticanus Latinus 1873, was perhaps originally composed in thirty-one books, of which only the final eighteen (14-31) have actually survived.  These treat the quarter century between 353 (appointment of Gallus and Julian as Caesares, as well as the beginning of Ammianus’ military career) and 378 (the year of the Battle of Adrianople, in which the emperor Valens and two thirds of his army were annihilated by Goths).  But since at the end of his work (31.16.9) the author himself affirms that he has begun, in his exposition of events, a principatu Caesaris Nervae (that is, from the point at which the narrative of Tacitus had broken off, and of which, therefore, Ammianus proposes himself as a continuator), it remains uncertain if the first thirteen books of the Res gestae, which are no longer extant, covered more than two hundred fifty years of history (AD 96-353) in brief, as the majority maintains, or rather if Ammianus had composed, as has been proposed (Michael, Rowell), two distinct historical works, the first (from the principate of Nerva through Constantine) completely lost, and the second, having come to us without its first thirteen books, dedicated to the period comprising the ascension (306) or the death (337) of Constantine and the final confrontation between Constantius II and Magnentius (353).  It has further been proposed (Barnes) that the work of Ammianus was unified, but originally comprised thirty-six books, of which the final eighteen remain extant, while the initial eighteen have been lost; on this model, the first six would have been dedicated to a brief overview of the Roman empire from Nerva to Diocletian, and the remaining twelve to the age of Constantine and the Constantinidae.  The writing of the Res gestae, on which Ammianus worked for a full thirty years of his life, was probably undertaken more intensely during the fifteen years of 380-95, during which the author, having settled at Rome, had the opportunity to exploit the rich literary and archival documentation of the city, performing public readings of his work and enjoying success above all among the high functionaries and refined imperial bureaucrats, if we can give credence to the aforementioned letter of Libanius (1063 Förster) dating to 392 (Matthews, Frakes, Viansino, Rohrbacher).

Given the silence of the historian in this regard, it is likewise difficult to determine his sources: most likely diaries of war, letters, panegyrics, and perhaps the lost History of Eunapius, in addition to notices derived from his personal experience and from information gathered from the protagonists of the events described, such as Ursicinus and Julian.  Heir of the great historiographical tradition - and of classical literature in general - Ammianus reveals that he was familiar with, and made his own, the instruction of Thucydides and Polybius, with whom he has in common the investigation of the truth, an interest in autopsy (through testimony at first hand and the report of great political and military events), and the decision to avoid minutiae (which he does not, however, always renounce completely).  But nor he does forget the inheritance of Herodotus (and Sallust), inserting in his own narration full, juicy comments of a biographical nature, and also numerous excursus of a geographical, ethnographical, and sometimes even paradoxographical flavor, though not without strong interest in magic, astrology, and the occult.  But Tacitus, in particular, is the model of reference, from the end of whose Historiae, as was mentioned, Ammianus begins his own work.  He shared with his predecessor (as likewise with Livy) not only the moralistic and dramatic arrangement of the narration and the psychological characterization of protagonists in tones that were sometimes dark, but also admiration for Rome and her empire (which he still considers aeterni) and disdain for those who provoked for her a profound moral crisis, which he describes in bitter and strongly pessimistic tones.  Thus Ammianus’ is a history that eclectically collects and synthesizes many styles of the earlier historiographical and literary traditions - both Greek and Latin; but it is also a history, in the cultural panorama of the 4^th c. AD, that is struggling against the tide of the already wide circulation of breviaries, epitomes, and chronographic works that were prepared (some on the orders of the emperor Valens himself) to bridge severe gaps in the knowledge of a newly-emerging political and bureaucratic class that was often as ignorant as it was corrupt and inefficient.  The critical attitude assumed towards this production has earned Ammianus the well-known moniker, “the lonely historian” (Momigliano).

Although the Res gestae, as has been said (Sabbah), does not eschew narration of an autobiographical sort concerning adventures of the historian in the guise of a young official, it above all dwells at length on the person and the concerns of the emperor Julian, sketching a biography of him that sometimes flows into open panegyric.  The young emperor is presented in the guise of a “new Alexander,” who opposes the Persians and spreads Greek paideia among the barbarians; thanks to his intellectual, moral, and military gifts, he is considered capable of arresting (or perhaps better of slowing) the destructive process that Rome seems already to have inescapably begun, as is suggested by the appearance of the Huns and Goths in the final book of the work, in a dramatic fresco of history designed to take on universal significance.  The historian, however, shows that he did not share Julian’s less tolerant political choices: in particular the cruel and bigoted abhorrence for Christianity, which he rather views with a certain respect, describing it as a religio absoluta et simplex (21.16.18) that nihil nisi iustum suadet et lene (22.11.5), without, however, failing to judge it sometimes with irony - though never too obviously - as was common among numerous pagans at the end of the 4^th c. (Kelly).

In fact, the central theme par excellence of Ammianus’ Histories is the Roman Empire, which the historian views with a clear and genuine optimism, even after the defeat of Adrianople, as a political organism still far from its proper end (Matthews).

The autobiographical flavor of the work’s conclusion­ - a sort of sphragis - is highly controversial; through this the historian addresses his reader, describing himself as miles quondam et Graecus, but he also addresses his successors, exhorting them to procudere linguas ad maiores… stilos.  However one wishes to interpret them (Matthews, Blockley, Barnes, Kelly), Ammianus’ enigmatic affirmations constitute a sort of prescient caesura between his own work, which is the last to insert itself in the tradition of the great pagan historiography in the Greco-Latin tongue, and those of his successors, which were incapable of following his example. [G. Vanotti; tr. C. L. Caterine].