[AntiGuv]

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. Even if they failed, the conquered territories would've been impossible to administer if they resisted, and such resistance would've made a Byzantine reconquest inevitable very soon afterward.

The problem was that the Muslims treated the Coptic and Syriac Christians better than the Byzantine authorities had in a long while. The rest is history!

[RobbyS]

For a long time the Copts and Syrians thought of the Muslims as just another Christian sect. And for a long time the Arabs kept themselves apart from the "infidels." Cairo began as an Arab camp.

[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.