Early Muslim conquests
The early Muslim conquests or early Islamic conquests, also known as the Arab conquests, were a series of wars initiated in the 7th century by Muhammad, the prophet of Islam. He established the First Islamic state at Medina in Arabia, that expanded rapidly under the succeeding Rashidun Caliphate and Umayyad Caliphate, culminating in Muslim rule being established in Asia, Northern Africa, and Southern Europe over the following century. According to historian James Buchan: "In speed and extent, the first Arab conquests were matched only by those of Alexander the Great, and they were more lasting." At their height, the territory that was conquered by the Arab Muslims stretched from the Iberian Peninsula in the west to India in the east; Muslim control spanned Sicily, most of the Middle East and North Africa, and the Caucasus and Central Asia.
Among other drastic changes, the early Muslim conquests brought about the collapse of the Sasanian Empire and great territorial losses for the Byzantine Empire. Explanations for the Muslim victories have been difficult to discover, primarily because only fragmentary sources have survived from the period. American scholar Fred Donner suggests that Muhammad's establishment of an Islamic state in Arabia coupled with ideological coherence and mobilization constituted the main factor that propelled the early Muslim armies to successfully establish, in the timespan of roughly a century, one of the largest empires in world history. Estimates of the total area of the combined territory held by the early Muslim empire at the conquests' peak have been as high as. Most historians also agree that, as another primary factor determining the early Muslim conquests' success, the Sasanians and the Byzantines were militarily and economically exhausted from decades of warfare against each other.
It has been suggested that Jews and some Christians in Sasanian and Byzantine territory were dissatisfied and welcomed the invading Muslim troops, largely because of religious conflict in both empires. However, confederations of Arab Christians, including the Ghassanids, initially allied themselves with the Byzantines. There were also instances of alliances between the Sasanians and the Byzantines, such as when they fought together against the Rashidun army during the Battle of Firaz. Some of the lands lost by the Byzantines to the Muslims had been reclaimed from the Sasanians only a few years prior to the Muslim conquests.
Background
Pre-Islamic Arabia
Arabia was a region that hosted several cultures, some urban and others nomadic Bedouin. Arabian society was divided along tribal and clan lines, with the most important divisions being between the "southern" and "northern" tribal associations. Both the Byzantine and Sasanian empires competed for influence in Arabia by sponsoring clients; in turn, Arabian tribes sought the patronage of the two rival empires to bolster their own ambitions. The Lakhmid kingdom, which covered parts of what is now southern Iraq and northern Saudi Arabia was a client of Persia, and in 602 the Persians deposed the Lakhmids to take over the defense of the southern frontier. This left the Persians exposed and overextended, helping to set the stage for the collapse of the Persian Empire later that century.Southern Arabia had, for thousands of years, been a wealthy region at the center of an international trading network, prominently including the spice trade, linking Eurasia, Africa Europe, the Middle East, India and even from as far away as China. In turn, the Yemeni were skilled sailors, travelling up the Red Sea to Egypt and across the Indian Ocean to India and down the east African coast. Inland, the valleys of Yemen had been cultivated by a system of irrigation that had been set back when the Marib Dam was destroyed by an earthquake in about 450 AD. Frankincense and myrrh had been greatly valued in the Mediterranean region, being used in religious ceremonies. However, the conversion of the Mediterranean world to Christianity had significantly reduced the demand for these commodities, causing a major economic slump in southern Arabia which helped to create the impression that Arabia was a backward region.
Byzantine–Sasanian Wars
The prolonged and escalating Byzantine–Sasanian wars of the 6th and 7th centuries and the recurring outbreaks of bubonic plague left both empires exhausted and weakened in the face of the sudden emergence and expansion of the Arabs. The last of these wars ended with victory for the Byzantines: Emperor Heraclius regained all lost territories and restored the True Cross to Jerusalem in 629. The war against Zoroastrian Persia, whose people revered fire as a symbol of divine power, had been portrayed by Heraclius as a holy war in defense of the Christian faith and the Wood of the Holy Cross, as splinters of wood said to be from the True Cross were known, had been used to inspire Christian fighting zeal. The idea of a holy war against the "fire worshipers", as the Christians called the Zoroastrians, had aroused much enthusiasm, leading to an all-out effort to defeat the Persians.Nevertheless, neither empire was given any chance to recover, as within a few years they were overrun by the advances of the Arabs, which, according to James 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".
Conquest of the Arabian Peninsula
According to Islamic tradition, Muhammad, a merchant from Mecca, began receiving revelations from God through the archangel Gabriel, emphasizing the worship of one God and social justice. After entering conflict with and persecution by the elite of Mecca, he migrated to the city of Yathrib, where he established the first Islamic state. By 630, he and his followers returned to and conquered Mecca. Already, late in Muhammad's life, the first Muslim-Byzantine skirmishes took place, such as during the Battle of Mu'tah in 629. In 632, Muhammad died and was succeeded by the first caliph Abu Bakr, who united the Arabian Peninsula after the Ridda Wars.Factors promoting the success of the conquests
Many of the material conditions of the pre-Islamic Arabian Peninsula and ancient Near East enabled the success of the conquests. First, over the course of the sixth and early seventh century, all major state and military powers of the Arabian Peninsula dissolved, leaving behind a power vacuum. For the Byzantine and Persian empires, they each respectively projected power into the Arabian peninsula through alliances with two major Arab kingdoms, the Ghassanids and the Lakhmids. The main function of these kingdoms, for the empires, was to serve as buffer states protecting the empires from military incursions by Arab nomads, and to wage proxy wars against each other. However, in the late sixth century, both empires sought direct control over their borders with the Peninsula, and deposed their respective client kingdoms in the process, a few years before the start of the Byzantine-Sasanian wars. The only other principal military force on the Peninsula, the Himyarite Kingdom of South Arabia, had already dissolved by the middle of the sixth century, due to a combination of military, climactic, and economic factors.By the mid-630s, in the years immediately before the invasion of the Near East, the now-united Arabian Peninsula had achieved its greatest power relative to that of the Near Eastern forces. The Byzantine and Persian powers were unusually weak at this time, as a consequence of a catastrophic and full-fledged thirty-year war between the two that had depleted their manpower, resources, and morale. By contrast, the Arabian Peninsula had been united under a singular leadership for the first time. Accordingly, "the military energy of the Arabs, hitherto dissipated in low-level conflicts between tribal groups, could now be redirected to the conquest of the vastly richer lands beyond the Arabian borders, catching the empires at a time when they were least able to parry this threat. In short, if the Arabs were ever to conquer themselves an empire, this was when it had to happen."
Historians have compared the early Muslim conquests to other conquests. According to Robert Hoyland, lightning-speed conquests over large regions have historically been achieved by nomadic groups found beyond established civilizations, like the Mongols. McCants compares the Arab conquests to earlier conquests of the Near East, first by Alexander the Great of Macedon, and then by the Romans. Between the Macedonians, the early Romans, and the early Arabs, McCants says: "All three conquering peoples hailed from the margins of the Near East, and their conquests united separate, complex societies with ancient native traditions of learning and legitimation distinct from those of the conquerors and one another". In the decades before the Arab conquests by Muhammad and his successors, most of the Arabian Peninsula was previously united by the conquests of the South Arabian king Abraha, including almost all of Arabia Deserta, with multiple campaigns being launched into the Hejaz and even into southern Iraq. Abraha's power at this time is further reflected by his hosting of an international conference which included ambassadors being sent for attendance from across the Byzantine, Persian, Ghassanid, and Lakhmid powers. In the 600s and 610s, the region of the Near East north of the Arabian Peninsula, was already under a constant fear and threat of Arab invasion. Already in the mid-6th century, the Romans had to sign a peace treaty that involved paying tribute to Arab kingdoms so as not to be attacked by them. The Battle of Dhi Qar, taking place between 604 and 611, already resulted in the an Arab tribal confederation, the Banu Bakr, defeating the Sasanians in southern Iraq and reducing Sasanian control over Eastern Arabia.