It was the duty of the faithful to conquer as much of the world as possible for Islam... The first conquests of the Arabs began as mere raids for plunder, and only turned into permanent occupation after experience has shown the weakness of the enemy... The Arabs, although they conquered a great part of the world in the name of a new religion were not a very religious race; the motive of their conquests was plunder and wealth rather than religion.
Bertrand Russell, The History of Western Philosophy, Book Two, Part 2, Chapter X: Mohammedan Culture