Some aspects of the transformation of Byzantium in the years of the Heraclian dynasty

Why did the Byzantine empire not succaumb to the various forces, internal and external, which during the seventh century threatened to destroy it? The question has often exercised the minds of historians. some have seen its survival as mere accident, the failure of its foes adequately to organise their efforts at conquest or the result of unavoidable internal divisions within the caliphate. Others have seen the impregnable position of Constantinople, the queen of cities, as the key: yet others have regarded the strength of Orthodox Christianity and the cultural bonds it forged as a crucial factor; while some historians have seen the well-structured and flexible administrative, fiscal and military apparatuses of the state as the foundation of its survival. All of these - although I should wish to modify each statement in different ways - played a role, of that there can be little doubt. But to look for single causes, or indeed prime movers, is to misunderstand the very nature of historical change. For in many ways the late Roman state did not survive, at least not in the sense that protagonists of a "continuity" approach to the problem would have us believe. The physical space - albeit much reduced - the geography and climate (with natural and usually very gradual shifts) remain much the same. But late Roman urban culture vanishes entirely, along with much of the cultural baggage it carried with it. Instead, new systems of thought develop, new approaches to art and representation are refined, new administrative structures are evolved. Power relationships within the ruling elite also change - the old senatorial establishment, with much of the literary culture associated with it, disappears, to be replaced by a very different elite, of different social, cultural and often ethnic origins. Those aspects of the traditional elite culture that did survive came to play a different role in the ideological world of this new class, although there is no reason to doubt that this new medieval elite included elements of the older establishment.

J. F. Haldon, Byzantium in the Seventh century: The transformation of a culture (Cambridge 1991), pp. 443-4.

The introduction of the thematic system

At this point, our meagre sources begin to use Hellenized names for the empire's five main armies and the districts where they were stationed. Both the armies and their districts are called "themes" (themata), and their commanders strategoi ("generals", except for the Count of the Opsician Theme). The origin of these themes is not in much doubt: they were evidently the field armies of the previous period. The large Opsician Theme combined what had been the two Praesental armies, while the Anatolic, Armeniac, and Thracesian themes were the former armies of the East, Armenia, and Thrace, which had retreated from their original positions to new stations in Anatolia. Only the origin of the Carabisian Theme is doubtful; the best guess is probably that its men, who were marines, came from the former army of Illyricum.
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Besides saving the government desperately needed cash, giving the soldiers land grants had other consequences, one advantageous and one not. The advantage was that, with themes now covering almost the whole empire, every important region had resident soldiers to defend it, who were strongly motivated to defend their own land. The disadvantage for the government was that, once the soldiers became mostly self-supporting, they had less reason to obey the emperor, and were easier to raise in rebellion against him. Though Constans II is unlikely to have anticipated either of these effects, he soon saw both of them worked.

Warren Treadgold, «The struggle for survival (641-780)», in C. Mango (ed.), The Oxford History of Byzantium (Oxford-New York 2002), pp. 131-3.