Gibbon's relationship with Ammianus Marcellinus, by far the most important of his sources in the second instalment of The Decline and Fall, also reveals the transitional nature of Volumes II and III. The Englishman's attitude to his Roman guide betrays his anxieties and concerns about narrative, while revealing the change in his view of historiography from seeing it as an exercise in authoritative orchestration to appreciating it as an undertaking in which the control and composure of the historian are jeopardised. In the various use Gibbon makes of Ammianus, we find once more, as we found in the portrait of Julian, the vestigial allure of philosophic historiography together with an appreciation of all the aspects of these difficult centuries which ever more peremptorily challenge that idiom.

The reader of The Decline and Fall is likely to be most aware of Gibbon's reliance on ‘the military historian’ in a passage which occurs long after his most extended use of the Res Gestae (a section of The Decline and Fall bounded by Chapters XIX and XXVI).