Excerpts from Books and Wikipedia
The Abbasid dynasty that began in 750 represented a shift in power away from the Arabs and toward the Persians. In 762 the second Abbasid caliph moved the capital to Iraq. The new city, Baghdad was built in four years. By the end of the century, it was the largest city in the world and shined in the golden age of Islam. [Furtado: 1001 Days]