Book 26 of Ammianus' Res Gestae is the first of the hexad which deals with the rule of the emperors Valentinian and Valens (364-378). In the first five chapters Ammianus describes the election of Valentinian, who appointed his brother Valens as his co-ruler, and subsequently divided the empire into an eastern and a western part. The next chapters deal with the revolt of Procopius. They offer the most detailed account of a coup d' etat in Roman historiography. The memory of Julian, whose death was the central theme of the preceding book, is still very much alive. None of the three...