Heraclius, Persia and the Arab Conquests
In my recent articles "Fundamentalist Islam's Cargo Cult" and "Conversions", I mentioned the devastation caused by the final war between Rome and Persia as the key precondition that enabled the explosive military expansion of Islam. In a discussion that followed on Brin-L, somebody expressed the opinion that it was bureaucracy and religious divisions that caused to the decline and fall of the Byzantine Empire and not the devastation caused by war with Persia. I think that the characterisation of the period as one of "decline and fall" is false - Byzantium endured for almost another thousand years and saw periods of renewed power - and also that bureaucracy and religious divisions were largely irrelevant to the conditions in the seventh century when the major Arabic conquests took place[1]. I'd like to say a little more about the reign of Heraclius, which in my view forms the natural break between "Roman" and "Byzantine" (although there were, of course, many continuities across that period too). Whereas Justinian's empire in the mid sixth century was manifestly Roman, and Justinian saw himself as the heir of Augustus and Diocletian, Heraclius clearly didn't. The near terminal crisis of the empire during his reign changed the entire character of the state. Furthermore, that era was one of the key turning points in world history.
For the whole period of the Dominate, from the end of the troubled third century until the final war between Rome and Persia, the military strategy of the Romans was dominated by the Persian frontier. Even during the period of the fall of the western part of the Empire, the bulk of Roman forces were tied up in the east. (Indeed, if not for this the western provinces would almost certainly not have fallen, and if the threat of Persia had receded then the recovery of the west by Justinian's generals Belisarius and Narses would probably have been much more complete, plague or no plague.) For much of this period the massive Roman forces and fortifications along the frontier preserved the peace although there were limited wars in the buffer regions.
During the century and a half between the fall of the west and the final war, there were relatively small wars during 502-6, 526-32 and 540-57 (a more serious pair of overlapping wars on different fronts during which Antioch fell to the Persians). Then in 602, the apocalypse that the balance of military might between the two powers had postponed for centuries finally broke out. The Romans had been weakened by another bout of civil war, military unrest and the invasion of the Balkans by the Avars. The Persian king Khosrau II took advantage of this weakness and invaded Roman Mesopotamia. In 608, Heraclius, the son of the Exarch of Africa, rebelled against the emperor Phocas, whose rule had been generally disastrous, and took Constantinople in 610. The renewed civil war in the Roman Empire further strengthened the position of the Persians, who invaded Syria, taking Damascus in 613 and Jerusalem in 614 and then conquering Egypt in 616 (it remained under Persian control for a decade). At the low point for the Romans, the territory not under enemy occupation in the east was reduced almost to the city of Constantinople itself: the Avars controlled the Balkans and the campfires of the Persians were visible just across the Bosphorus. The imperial government came within a whisker of abandoning the city and moving the capital to the safety of Carthage in Africa.
I don't think anybody at the time can have expected anything except the imminent dissolution of the Roman Empire. Remarkably, that's not what happened, largely because of Heraclius himself. Unlike most of the later Roman emperors his charisma could inspire immense loyalty and courage in his troops and he turned out to be something of an organisational and military genius. He totally reformed the administrative and military structure of the Empire (and along the way replaced Latin with Greek as the official language of the imperial government). His reorganisation largely endured for eight centuries, which is why I consider him the first Byzantine emperor. Heraclius was also the first emperor to lead his troops in person for over two hundred years, and his campaigns between 621 and 627 were spectacular indeed. A combination of strategic and tactical brilliance and skillful exploitation of weaknesses in the Persian political system brought the Persian empire to its knees, plunging it into a series of crises that fatally weakened it. By the end of the war, the Romans had recovered all the territory they'd lost to Persia, but they were territories ravaged by a quarter of a century of foreign occupation and war.
It was only seven years after the end of this last war between Rome and Persia that the armies of Islam erupted from Arabia. By that time Heraclius had fallen into terminal illness, and his generals failed him. Syria fell to the Arabs in 634, the Persian army was defeated in 636, Armenia and Egypt were conquered in 639, Africa in 642, Persia itself in 651...
[1] However, religious divisions amongst Christians in the Middle East were certainly important after the Islamic expansion. Many Monophysite Christians found their new Muslim rulers much more agreeable than the Byzantine emperors who viewed them as heretics.