The so-called barbarian conquest of Rome led the invaders to convert to the religions of the conquered. The Mongols, who butchered millions of people, second in Barbarity to Westerners only perhaps. There're clear proofs that Westerners cooperated so much with Mongols to destroy Muslims that Mongol leaders were expected to convert to Christianity. Unfortunately, they converted to islam and never repeated their history of barbarity in cooperation of West.
Abdal Hakim Murad, while mentioning these two conquests, mentions how simple, unsophisticated noblemen of Arab in a blink of eye spread through most cultured and sophisticated lands of their times and instead of getting converted led the conquered to convert, although over centuries because they didn't point swords at their throats. It's nothing short of a miracle, he adds.