Bernd 08/14/2018 (Tue) 18:53:55 No.18480 del
So just to be clear we're talking about the years between 622-788? Or 632-711 or other dates? Not that it matters much.

I believe multiple reasons stand behind their success, in this post I want to put forward one: their ability to winning people over. Starting with Muhammad who got the Bedouins - the main fighting force of Arabia - to do his bid during the initial conquests. Then later they get over the people of Mesopotamia, the first force which added the capability of employing real sieges. Then later they get the Berbers to conquer most of Iberia for them, etc.
Persuasion has three ways: threat of violence, buying them, con them through their beliefs or customs. The Islamic conquerors used all three successfully.
I think even the rise of the Caliphate (both the state and the position) was a necessity which was pressured by the need of the conquered people and their customs, usual way of life. They got used to living under emperors both in the west (Byzantium) and east (Persia), they gave the idea to the Muslim leadership and it became the means for controlling the newly acquired regions with it's familiarity to the folks living there. Not everyone converted to Islam but these understood and accepted the concept of emperor so they could be controlled by lay authority even if religious authority did nothing for them.

Little bit different but related aspect: foreign leaders understood the concept of an emperor as well, it was better than some inherited prophetic title or whatever. Especially if their foreign leaders gave crap about his prophetic nature.