Baghdad was the city that medieval Arabic geographers put in the center of the world. The history of Baghdad is divided into three phases, first, the prestigious capital of the Abbasid Caliphs from the time of its foundation in 762 by al-Mansûr up to its conquest by Mongol armies in 1258; then, for centuries, a simple provincial metropolis, and finally, since 1921, the capital of Iraq, whose dramatic present assails us with images of devastation. The Abbasid Caliphs took power in the aftermath of an important insurrection that overthrew the former Umayyad dynasty over the years 746-50. The palatial city founded by al-Mansûr has often been called the Round City because of its circular form. Ya‘qûbî affirms that it was the only round city known in the whole world. The city founded by al-Mansûr was transformed quickly as the result of the displacement and multiplication of the Caliph's places of residence.