By the end of the 10th century the Fatimid Caliphate had replaced the Abassids as the major Arab power; they halted the Byzantine reconquests although border conflicts continued.

The last of the wars between the Roman and Persian empires ended with victory for the Byzantines: Emperor Heraclius regained all lost territories, and restored the True Cross to Jerusalem in 629.

[3] In the late 620s, the Islamic Prophet Muhammad had already managed to unify much of Arabia under Muslim rule via conquest as well as making alliances with neighboring tribes, and it was under his leadership that the first Muslim–Byzantine skirmishes took place.

[10] The Byzantine response involved the collection and dispatch of the maximum number of available troops under major commanders, including Theodore Trithyrius and the Armenian general Vahan, to eject the Muslims from their newly won territories.

[11] Heraclius' farewell exclamation (according to the 9th-century historian Al-Baladhuri)[12] while departing Antioch for Constantinople, is expressive of his disappointment: "Peace unto thee, O Syria, and what an excellent country this is for the enemy!

[18] The fall of Alexandria extinguished Byzantine rule in Egypt, and allowed the Muslims to continue their military expansion into North Africa; between 643 and 644 'Amr completed the conquest of Cyrenaica.

[30] Then ibn Nafi "plunged into the heart of the country, traversed the wilderness in which his successors erected the splendid capitals of Fes and Morocco, and at length penetrated to the verge of the Atlantic and the great desert".

[32] As the historian Luis Garcia de Valdeavellano explains:[33] In their struggle against the Byzantines and the Berbers, the Arab chieftains had greatly extended their African dominions, and as early as the year 682 Uqba had reached the shores of the Atlantic, but he was unable to occupy Tangier, for he was forced to turn back toward the Atlas Mountains by a man who became known to history and legend as Count Julian.

As the first tide of the Muslim conquests in the Near East ebbed off, and a semi-permanent border between the two powers was established, a wide zone, unclaimed by either Byzantines or Arabs and virtually deserted (known in Arabic as al-Ḍawāḥī, "the outer lands" and in Greek as τὰ ἄκρα, ta akra, "the extremities") emerged in Cilicia, along the southern approaches of the Taurus and Anti-Taurus mountain ranges, leaving Syria in Muslim and the Anatolian plateau in Byzantine hands.

Constantine IV (r. 661–685) however used a devastating new weapon that came to be known as "Greek fire", invented by a Christian refugee from Syria named Kallinikos of Heliopolis, to decisively defeat the attacking Umayyad navy in the Sea of Marmara, resulting in the lifting of the siege in 678.

[54] From July 717 to August 718, the city was besieged by land and sea by the Muslims, who built an extensive double line of circumvallation and contravallation on the landward side, isolating the capital.

Their attempt to complete the blockade by sea however failed when the Byzantine navy employed Greek fire against them; the Arab fleet kept well off the city walls, leaving Constantinople's supply routes open.

Under the Macedonian dynasty, exploiting the decline and fragmentation of the Abbasid Caliphate, the Byzantines gradually went on the offensive, and recovered much territory in the 10th century, which was lost however after 1071 to the Seljuk Turks.

From 720/721 however the Arab armies resumed their expeditions against Byzantine Anatolia, although now they were no longer aimed at conquest, but rather large-scale raids, plundering and devastating the countryside and only occasionally attacking forts or major settlements.

On the Arab side, Cilicia was permanently occupied and its deserted cities, such as Adana, Mopsuestia (al-Massisa) and, most importantly, Tarsus, were refortified and resettled under the early Abbasids.

[61] This was still a time when Byzantium was fighting for survival, and "the frontier provinces, devastated by war, were a land of ruined cities and deserted villages where a scattered population looked to rocky castles or impenetrable mountains rather than the armies of the empire to provide a minimum of security" (Kennedy).

[35] In response to the renewal of Arab invasions, and to a sequence of natural disasters such as the eruptions of the volcanic island of Thera,[62] the Emperor Leo III the Isaurian concluded that the Empire had lost divine favour.

Only in the late 730s did the Muslim raids again become a threat, but the great Byzantine victory at Akroinon and the turmoil of the Abbasid Revolution led to a pause in Arab attacks against the Empire.

[70][71] As a symbol of the Caliph's ritual role as the leader of the Muslim community, they were closely paralleled in official propaganda by the leadership by Abbasid family members of the annual pilgrimage (hajj) to Mecca.

Wishing to emphasize his piety and role as the leader of the Muslim community, Caliph Harun al-Rashid (r. 786–809) in particular was the most energetic of the early Abbasid rulers in his pursuit of warfare against Byzantium: he established his seat at Raqqa close to the frontier, he complemented the thughur in 786 by forming a second defensive line along northern Syria, the al-'Awasim, and was reputed to be spending alternating years leading the Hajj and leading a campaign into Anatolia, including the largest expedition assembled under the Abbasids, in 806.

[75][76] Continuing a trend started by his immediate predecessors, his reign also saw the development of far more regular contacts between the Abbasid court and Byzantium, with the exchange of embassies and letters being far more common than under the Umayyad rulers.

With the support of Caliph Al-Ma'mun, Arabs under the leadership of Thomas the Slav invaded, so that within a matter of months, only two themata in Asia Minor remained loyal to Emperor Michael II.

[82] In 863 during the reign of Michael III, the Byzantine general Petronas defeated and routed an Arab invasion force under the command of Umar al-Aqta at the Battle of Lalakaon inflicting heavy casualties and removing the Emirate of Melitene as a serious military threat.

[83][84] Umar died in battle and the remnants of his army was annihilated in subsequent clashes, allowing the Byzantines to celebrate the victory as revenge for the earlier Arab sacking of Amorion, while news of the defeats sparked riots in Baghdad and Samarra.

The Byzantine civil wars had weakened the Empire's position in the east, and the gains of Nikephoros II Phokas and John I Tzimiskes came close to being lost, with Aleppo besieged and Antioch under threat.

Although he did not have the force to drive into Palestine and reclaim Jerusalem, his victories did restore much of Syria to the empire – including the larger city of Antioch which was the seat of its eponymous Patriarch.

Piers Paul Read writes that by 1025, Byzantine land "stretched from the Straits of Messina and the northern Adriatic in the west to the River Danube and Crimea in the north, and to the cities of Melitene and Edessa beyond the Euphrates in the east.

While fighting for survival against the Islamic armies, the Empire was no longer able to provide the protection it had once offered to the Papacy; worse still, according to Thomas Woods, the Emperors "routinely intervened in the life of the Church in areas lying clearly beyond the state's competence".

Kaegi concludes that scholars must also subject the Byzantine tradition to critical scrutiny, as it "contains bias and cannot serve as an objective standard against which all Muslim sources may be confidently checked".

[97] The range of non-historical Byzantine sources is vast: they range from papyri to sermons (most notable those of Sophronius and Anastasius Sinaita), poetry (especially that of Sophronius and George of Pisidia) including the Acritic songs, correspondence often of a patristic provenance, apologetical treatises, apocalypses, hagiography, military manuals (in particular the Strategikon of Maurice from the beginning of the 7th century), and other non-literary sources, such as epigraphy, archeology, and numismatics.