That's the funny thing about armies, if you don't pay them they may go to work for somebody else.
I know, you're one of those guys who believes no Christian would willingly become a Moslem. Unfortunately that still goes on today ~ a couple of them were in on that terror threat the other day directed at airplanes.
There are numerous books out there concerning the rise of Islam ~ most of them written by non Moslems since it's so very risky for a Moslem to write something another Moslem might dispute. They consider all "differences of opinion" concerning the rise of Islam to be religious matters worthy of getting your throat cut.
So, you're safe in reading most of the books about that period of time ~ they are written mostly by Christians.