How did the Byzantine and Sasanian wars help enable early Muslim expansion?

The early Muslim expansion was significantly enabled by the exhaustion of the Byzantine Empire and the Sasanian Empire, which had been locked in a century of intense, draining conflict.12

By the time the Arab conquests began, these two powers had depleted their resources and military strength through prolonged hostilities. The Sasanian Empire, in particular, suffered from internal instability and a succession of rulers following the devastating wars of Khosrow II, leaving its borders vulnerable. Similarly, the Byzantine Empire, despite the efforts of Emperor Heraclius to rebuild, was forced to concentrate its remaining defenses on Anatolia and Egypt after suffering major defeats, such as at the Battle of Yarmouk.324

Because the representatives of Byzantine and Sasanian rule on the northern borders of Arabia were not strong enough to resist, the newly united Arab forces were able to capitalize on this power vacuum to expand rapidly, securing territories and resources that had previously been under imperial control.15