When contemplating the varieties of empire invoked in Gibbon's Decline and fall and discussed in this book, a number of themes have emerged. Given the scale and complexity of Gibbon's achievement, John Matthews has suggested that it be seen not so much as a search for a cause of decline as an investigation of a sequence of circumstances; we should think of the concept of decline and fall as a classical trope rather than as an analytical tool.

While there is much to recommend such a view, one might wish to make an exception of the first twelve chapters, in which the trope of decline does also seem to incorporate an underlying formula of the kind, which, following the work of Pocock, we have learned to call Machiavellian: a people, corrupted by conquests and luxury, loses its virtue and with it its liberty, handing over its defence to barbarian auxiliaries who sell the empire to the highest bidder. This theme, classic, as Pocock has shown, in eighteenth-century political analysis and rhetoric, appears in Gibbon less frequently thereafter, though it tends to recur whenever there is a discussion of the relation of conquered peoples to their conquerors. Such a relation is one in which each is fatal to the other: Goths and Vandals are subject in turn to corruption by their corrupt Roman subjects, Arabs by Persians, Mongols by Chinese, while the Turks come to play, in relation to the Arabs, a role analogous to that of the Praetorian Guard in Rome; Gibbon himself explicitly noted the parallels.