Why were the Byzantine and Sasanian empires vulnerable to Arab conquest in the 7th century?

The Byzantine and Sasanian empires were vulnerable to Arab conquest in the 7th century due to a combination of exhaustion, internal instability, and the collapse of regional security structures. Several key factors contributed to this vulnerability:

• Exhaustion from Prolonged Warfare: Both empires had been locked in intense, recurrent conflict for a century, which drained their military and economic resources. By the time the Arab conquests began, both states were significantly weakened by these exhausting hostilities.12
• Demobilization and Lack of Defense: Prior to 632, both empires had demobilized their forces, leaving them ill-prepared for the new threat. Furthermore, they had ceased supporting the client states on the frontiers of the Arabian Peninsula that had previously served as a buffer against Bedouin incursions.3
• Religious Disaffection: In the Byzantine Empire, religious controversy weakened the loyalty of subjects in provinces like Syria and Egypt. Attempts by the emperor Heraclius to impose religious compromises, such as monothelitism, failed to placate the local populations, who often found the subsequent Arab rule more tolerant than that of Constantinople.3
• Institutional Strain: The empires had faced earlier shocks, such as the 6th-century bubonic plague, which caused high mortality rates, disrupted labor, and hindered military effectiveness.45

Ultimately, the representatives of Byzantine and Sasanian rule on the northern borders of Arabia were not strong enough to resist the organized and purposive expansion of the early Muslim community.13