A human tsunami hit two exhausted empires after 622
From Medina’s first Islamic polity in 622, Muslim armies erupted from Arabia in wars begun under Muhammad and continued by his successors. Within decades they toppled the Sasanian Empire and shoved Byzantium back into Anatolia and the Balkans—a rupture historians still struggle to explain fully.
Arabia mixed urban and Bedouin worlds split by southern versus northern tribal blocs, while Byzantium and Persia sponsored rival clients. Southern Yemen had long sat on spice routes linking Eurasia, Africa, and even China, breeding skilled Red Sea and Indian Ocean sailors. That commercial depth, suddenly yoked to religious-political unity, faced two superpowers already bled white.
Sixth- and seventh-century Byzantine–Sasanian wars plus Justinianic plague left both giants exhausted. Heraclius’s hard-won restoration barely cooled before Arab forces—James Howard-Johnston’s human tsunami—overran the board. George Liska argued the endless Roman–Persian duel itself opened Islam’s road.
Muhammad, a Meccan merchant, clashed with the city's elite, moved to Yathrib, later Medina, and by 630 had returned to take Mecca; the first skirmish with Byzantium came at Mu'tah in 629. Conditions favoured what followed. Arabia's own states had dissolved during the sixth and early seventh centuries, leaving a vacuum, and the Ghassanids and Lakhmids, client kingdoms through which Byzantium and Persia projected power, faded. By the mid-630s the peninsula stood united under one leadership for the first time, just as a thirty-year war had drained its neighbours of men, money and morale.
Explaining the speed of victory is hard because so few sources survive. Robert Hoyland compares it to other lightning conquests by nomads from beyond settled civilisations, such as the Mongols, while McCants sets the Arabs beside Alexander's Macedonians and the Romans as invaders from the margins of the Near East. Some Jews and Christians under both empires may have welcomed or shrugged at the newcomers, though the Christian Ghassanids first sided with Byzantium. Arab fighters prized Indian steel swords, and Bedouin archers usually fought on foot. By the Umayyad era a standing army was paid monthly through a war ministry.
Source: Early Muslim conquests
