Excerpts from Books and Wikipedia
"Persianate customs were broadly adopted . . . and they began patronage of artists and scholars. Baghdad became a centre of science, culture, philosophy and invention in what became known as the Golden Age of Islam." Invasions of Baghdad by Iranian Buyids (945) and Seljuq Turks (1055) ended the power of the caliphs. "Due to the First Fitna which led to the sectarian division of Sunni vs. Shia Islam, the succession of Muhammad is disputed within Islam. The only two caliphs recognized in both Sunni and mainstream Shia Islam are Ali ibn Abi Talib and Hasan ibn Ali, considered the fourth and fifth or the first two, respectively." [Wikipedia] "While Charlemagne in his halting and stultified Latin was being crowned Holy Roman Emperor in 800, the Abbasid caliphs were already well into the monumental translation project that brought the Greek philosophical and scientific tradition into Arabic. Continuous traffic between Cordoba and Baghdad meant that the Andalusians were soon enough reading the same things and eagerly keeping up with the latest innovations, fashions, and products, and, eventually, capable of sending their own back in return." [Menocal: Ornament of the World, p. 82]