The Arab–Byzantine wars were a series of wars between the mostly Arab Muslims and the Byzantine Empire between the 7th and 11th centuries AD, started during the initial Muslim conquests under the expansionist Rashidun and Umayyad caliphs in the 7th century and continued by their successors until the mid-11th century.
The emergence of Muslim Arabs from Arabia in the 630s resulted in the rapid loss of Byzantium's southern provinces (Syria and Egypt) to the Arab Caliphate. Over the next fifty years, under the Umayyad caliphs, the Arabs would launch repeated raids into still-Byzantine Asia Minor, twice besiege the Byzantine capital of Constantinople, and conquer the Byzantine Exarchate of Africa. The situation did not stabilize until after the failure of the Second Arab Siege of Constantinople in 718, when the Taurus Mountains on the eastern rim of Asia Minor became established as the mutual, heavily fortified and largely depopulated frontier. Under the Abbasid Empire, relations became more normal, with embassies exchanged and even periods of truce, but conflict remained the norm, with almost annual raids and counter-raids, sponsored either by the Abbasid government or by local rulers, well into the 10th century.
During the first centuries, the Byzantines were usually on the defensive, and avoided open field battles, preferring to retreat to their fortified strongholds. Only after 740 did they begin to launch their raids in an attempt to combat the Arabs and take the lands they had lost, but still the Abbasid Empire was able to retaliate with often massive and destructive invasions of Asia Minor. With the decline and fragmentation of the Abbasid state after 861 and the concurrent strengthening of the Byzantine Empire under the Macedonian dynasty, the tide gradually turned. Over a period of fifty years from ca. 920 to 976, the Byzantines finally broke through the Muslim defences and restored their control over northern Syria and Greater Armenia. The last century of the Arab–Byzantine wars was dominated by frontier conflicts with the Fatimids in Syria, but the border remained stable until the appearance of a new people, the Seljuk Turks, after 1060.
The Arabs also took to the sea, and from the 650s on, the entire Mediterranean Sea became a battleground, with raids and counter-raids being launched against islands and the coastal settlements. Arab raids reached a peak in the 9th and early 10th centuries, after the conquests of Crete, Malta and Sicily, with their fleets reaching the coasts of France and Dalmatia and even the suburbs of Constantinople.
The prolonged and escalating Byzantine–Sassanid wars of the 6th and 7th centuries and the recurring outbreaks of bubonic plague (Plague of Justinian) left both empires exhausted and vulnerable in the face of the sudden emergence and expansion of the Arabs. 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.
Nevertheless, neither empire was given any chance to recover, as within a few years they found themselves in conflict with the Arabs (newly united by Islam), which, according to Howard-Johnston, "can only be likened to a human tsunami". According to George Liska, the "unnecessarily prolonged Byzantine–Persian conflict opened the way for Islam".
In 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. Just a few months after Emperor Heraclius and the Persian general Shahrbaraz agreed on terms for the withdrawal of Persian troops from occupied Byzantine eastern provinces in 629, Arab and Byzantine troops confronted each other at the Mu'tah in response to the murder of Muhammad's ambassador at the hands of the Ghassanids, a Byzantine vassal kingdom. Muhammad died in 632 and was succeeded by Abu Bakr, the first Caliph with undisputed control of the entire Arabian Peninsula after the successful Ridda Wars, which resulted in the consolidation of a powerful Muslim state throughout the peninsula.