The year 629 marked a turning point when Emperor Heraclius restored the True Cross to Jerusalem after decades of war with Persia. Yet this victory offered no respite for either empire. The Plague of Justinian had already ravaged populations across both realms, leaving them vulnerable to new threats. Within years, Arab forces emerged as a unified power under Islam, described by historian Howard-Johnston as resembling a human tsunami. The prolonged conflict between Rome and Persia had opened the door for Islamic expansion. In the late 620s, Prophet Muhammad had already united much of Arabia through conquest and alliances. Just months after Persian troops withdrew from Byzantine provinces in 629, Arab and Byzantine armies clashed at the Battle of Mu'tah following the murder of Muhammad's ambassador.
Rapid Conquests
In 634, Rashidun Caliphate forces achieved a decisive victory near Ajnadayn against a Byzantine army composed of imperial troops and local levies. Monophysites and Jews throughout Syria welcomed the Arabs as liberators due to their discontent with Byzantine rule. By 637, Muslim forces captured Damascus under Khalid ibn al-Walid's command. The fall of Alexandria in September 642 extinguished Byzantine rule in Egypt, depriving the Empire of its valuable wheat supply. This loss caused food shortages throughout the Byzantine Empire and weakened its armies in subsequent decades. Between 643 and 644, 'Amr completed the conquest of Cyrenaica while Uthman succeeded Caliph Umar after his death. The Islamic forces raided Sicily in 652, capturing Cyprus and Crete in 653 before Crete reverted to Eastern Roman control until the 820s.