In his book "Negation in India" Famous Belgian historian Koenraad Elst wrote:
The Blitzkrieg of the Muslim armies in the first decades after the birth of their religion had such enduring results precisely because the Pagan populations in West- and Central-Asia had no choice (except death) but to convert. Whatever the converts' own resentment, their children grew up as Muslims and gradually identified with this religion. Within a few generations the initial resistance against these forcible converions was forgotten, and these areas became heidenfrei (free from Pagans, cfr. judenfrei).