The Arab conquests of the seventh century were the principal external catastrophe of medieval Greek-Christian Christendom, and the event from which the eastern Empire emerged in substantially the form — territorially Anatolian and southern-Balkan, demographically Greek-speaking, doctrinally Orthodox — that it would maintain for the next eight hundred years. Within a single generation between Muhammad's death at Medina in 632 and the Umayyad consolidation in the 660s, the empire lost Syria, Palestine, Egypt and the North African provinces that the Justinianic reconquest had only recently re-incorporated. The first major Arab siege of Constantinople in 674–678 was held off; the second in 717–718 was held off again. Beyond those defensive successes lay the substantive recognition that the late-antique Mediterranean order had ended.
The exhausted Empire
The eastern Empire that the Arab armies confronted in the 630s was substantially exhausted by the most recent of its perennial wars with Sasanian Persia — the war of 602–628, prosecuted by Heraclius against Khosrow II, which had produced an initial Persian victory (the loss of Syria, Palestine and Egypt to Persian armies between 614 and 619, the Persian capture of the True Cross at Jerusalem in 614, the Persian siege of Constantinople in 626 in concert with the Avars) and a substantial Roman counter-stroke (Heraclius's invasion of Mesopotamia in 627, the Battle of Nineveh in December 627, the Persian internal collapse and the restoration of the lost provinces in 628–629). The exhausted recovery of 628–632 had not produced the substantial reorganisation of the imperial army and finances that the situation required. When Arab raiding forces appeared on the southern frontier of Palestine in 633–34, the imperial response was the dispatch of substantial but tactically mishandled field armies.
Yarmouk
The decisive engagement was the Battle of Yarmouk in August 636 — six days of substantial fighting in the upper Yarmouk valley (southern Syria, south-east of the Sea of Galilee) between an Arab field army under Khalid ibn al-Walid (approximately twenty-five thousand men) and an imperial field army under Vahan and Theodore the Sakellarios (approximately forty thousand men, including Armenian, Slavic, Lakhmid and Ghassanid auxiliaries). The Arab tactical victory — substantially attributable to Khalid's operational manoeuvre, to the substantial cohesion of the Arab forces under recent religious-political mobilisation, and to the substantial weaknesses of the multi-ethnic imperial command structure — produced the complete destruction of the imperial Syrian field army and the substantial collapse of the Roman administrative position in Syria within eighteen months. Damascus had already surrendered in September 634; Jerusalem fell in 637, Antioch in 638, the remaining Syrian and Palestinian cities through 639–640. The Egyptian campaign under 'Amr ibn al-'As proceeded in 640–642: the battle of Heliopolis in July 640, the surrender of the great fortress of Babylon-in-Egypt in April 641, the capitulation of Alexandria in November 642. The North African provinces fell progressively over the subsequent half-century, culminating in the Arab capture of Carthage in 698.
Why the East fell
The substantial historiographical question has been the apparent ease of the conquests. The principal contributing factors that the literature has substantially canvassed: the exhaustion of the imperial military by the immediately preceding Persian war; the substantial fiscal depletion of the imperial treasury; the disaffection of the substantial Monophysite Christian populations of Syria and Egypt from the imperial Chalcedonian Orthodoxy (whose persecution under Heraclius and his successors had substantially alienated the provincial populations); the relative novelty and high morale of the Arab forces; the substantial Arab strategic skill in successive operational deployments; the relative quality of the leadership on both sides. The substantial verdict of recent historiography — particularly James Howard-Johnston's Witnesses to a World Crisis (2010) — is that the conquests were the consequence of a substantial military superiority of the Arab forces of the period, combined with the substantial weakness of the contemporaneous imperial military, and that the cultural-religious explanations (the Monophysite welcome of the Arabs) are substantially overstated.
The administrative reconstruction
The reduced empire of the post-642 period — substantially Anatolian, southern-Balkan, southern-Italian, with a few residual coastal positions in North Africa and the Aegean islands — underwent a substantial administrative reorganisation under Constans II (641–668) and Constantine IV (668–685). The substantial change was the institution of the thematic system: the substitution of substantial military-administrative provinces (themata) for the late-antique civil-and-military separation. Each theme was administered by a strategos with combined military and civilian authority; the themes raised their own regional armies, predominantly through grants of land to soldiers in return for military service; the substantial centralisation of administrative authority that had characterised the Justinianic empire was substantially relaxed in favour of provincial decentralisation. The thematic system was the substantial institutional adaptation that permitted the eastern Empire to survive on substantially diminished resources.
The first siege of Constantinople
The first sustained Arab attempt on the imperial capital was the siege of 674–678 — substantially a sequence of summer naval expeditions against the Marmara coast and the Bosphorus during four consecutive campaigning seasons, conducted by the Umayyad caliph Mu'awiya I. The Roman defence under Constantine IV produced one of the substantial technological breakthroughs of the medieval period: the introduction of Greek fire, an incendiary naval weapon (likely a petroleum-based mixture, the exact composition of which has been substantially debated) that could be projected from siphons mounted on Roman naval galleys and that proved substantially effective against the Arab fleet. The Arab withdrawal in 678 and the subsequent thirty-year truce (the Treaty of 679) substantially demonstrated the limits of the Arab military capacity against the substantial defensive resources of the imperial capital.
The second siege of 717–718, conducted by Maslama ibn Abd al-Malik on behalf of his brother the caliph Sulayman, was the substantial Arab attempt on Constantinople. The Arab forces comprised approximately a hundred thousand troops with a substantial naval contingent; the Roman defence under Leo III, who had seized the throne shortly before the siege opened, drew substantially on Greek fire (against the Arab fleet), on the Theodosian walls, on the substantial winter weather (which produced substantial Arab casualties from cold and disease), and on the diplomatic intervention of the Bulgarian khan Tervel (who attacked the Arab land forces from the north). The Arab withdrawal in August 718, with substantial fleet and army losses, was the substantial end of the period of Arab strategic expansion against the Empire. The frontier substantially stabilised along the Taurus Mountains for the next three hundred years.
The substantive outcome
The eastern Empire that emerged from the eighth century was substantially smaller — perhaps a third of the territory of the Justinianic empire at its peak — but it was substantially Greek-speaking, substantially homogeneous in its Orthodox Christianity, substantially adapted to its reduced economic and military resources through the thematic administrative system, and substantially defended by the natural frontier of the Anatolian plateau, the Aegean Sea and the Balkan mountains. The substantial cultural-religious self-identification of the Empire would shift, over the eighth and ninth centuries, from late-antique Roman to medieval Greek-Christian. The substantial dispute over religious imagery — the iconoclast controversy — that would dominate the next century is the subject of chapter IV.
End of Chapter III