How did conflicts between the Byzantine and Sasanian empires help early Muslim expansion?

The conflicts between the Byzantine Empire and the Sasanian Empire were a critical factor in the success of early Muslim expansion. For over a century, these two powers engaged in exhausting hostilities that depleted their military and economic resources, leaving their frontier regions vulnerable.123

By the time the Arab conquests began, both empires were significantly weakened by this prolonged warfare, as well as by internal political instability and social strife. For instance, the Sasanian Empire suffered from a rapid succession of rulers and internal dissent following the death of Khosrow II, leaving it ill-equipped to resist the emerging Muslim forces. Similarly, the Byzantine Empire had been devastated by the costs of these wars, forcing it to focus on defensive efforts in Anatolia and Egypt after losing key territories in Syria.4256

Because the representatives of Byzantine and Sasanian rule on the borders of Arabia were not strong enough to mount an effective defense, the nascent Caliphate was able to rapidly exert control over the Levant and Mesopotamia.127