Excerpts from Books and Wikipedia
"In 633, when the Sasanian king Yazdegerd III was ruling over Iran, the Muslims under Umar invaded the country right after it had been in a bloody civil war. . . . The Muslim conquest of Persia ended the Sasanian Empire and led to the eventual decline of the Zoroastrian religion in Persia. Over time, the majority of Iranians converted to Islam. Most of the aspects of the previous Persian civilizations were not discarded, but were absorbed by the new Islamic polity." [Wikipedia]