Excerpts from Books and Wikipedia
"The overthrow of the Umayyad dynasty by the Abbasids in 750 represented a shift of power in the Muslim caliphate away from Arab lands toward Persia. Because of this, the first Abbasid caliph, Abu al-Abbas, decided to abandon Damascus and move east to a temporary site on the banks of the Euphrates in Iraq. . . . Baghdad was completed in just four years. . . . Not surprisingly, Baghdad flourished and soon spread beyond its walls. By the end of the century it had become the world's largest city. . . . Baghdad played host to a golden age of Islamic cultural achievements." [1001 Days]