Caliph Al Mansur founded Baghdad as the new capital of his state on July 30, 762. Al Mansur ruled the Abbasid Caliphate, a vast state that stretched from North Africa to the borders of India and from the Caspian Sea and the Black Sea to the south of the Arabian Peninsula. The newly built city was circular in shape, and Al Mansur had a palace in its center.
Baghdad was built on a site along the Tigris River, at the part of its course where it is closest to the Euphrates River. The rivers provided plenty of water in the area with little rainfall, which was a prerequisite for the lives of large numbers of people in Baghdad. In addition, the proximity of the Euphrates and the Tigris ensured good transport links between Baghdad and the entire region.
Baghdad was founded north of the site of Babylon, a former large city center from ancient times. Near Baghdad were also the remains of the former great cities of Seleucia (Hellenistic capitals from the time of Alexander the Great’s successor) and Ctesiphon (Persian capitals from the time of the Sassanids). Baghdad eventually became the largest city in the world at the time, with more than a million inhabitants, at a time when the largest cities in Western Europe had only a few thousand inhabitants.