[Antoninus]

That's very true, but even with the depletion of the Byzantine imperial forces, the local Syrian and Egyptian militias should've been more than adequate to fend off the Muslims if they were inclined to do so.

What evidence do you have for the existence of such militias? Over and over again during the 6th and 7th centuries, the Persians invaded the Byzantine east and were completely unopposed unless an Imperial army happened to be in the area. The theme system hadn't been set up by the 630s. The provinces were simply incapable of defending themselves against an invading host--even a small one--without assistance from Constantinople.

[AntiGuv]

It's been a long while since I've read this history. Long while meaning I was in high school. In any event, here's the first link that came up on Google:

Greek colonists in the Middle East always formed a local militia when needed. In the last challenge to Greek rule, there were no Greek local militias available. They were killed in the last Persian war. Not only were the Byzantines exhausted, but their native troops deserted or turned against them. They were also without their local Greek auxiliaries.⁴

Native troops cannot desert or betray if they don't exist.......

⁴[Yarmuq - ]Alfred Guillaume, op. cit. p.19

[AntiGuv]

Oh, I forgot to include the actual link in that last post..