The city founded in 762 CE by the Abbasid caliph al-Mansur on the Tigris as a planned circular capital, which grew into one of the largest and richest cities in the medieval world and the center of the Islamic Golden Age, before the Mongol sack of 1258 and centuries as a provincial town — and which is now the capital of Iraq.
Baghdad was drawn as a perfect circle. The caliph al-Mansur laid out his new capital in 762 CE as a round city with the palace and mosque at its exact center, and within a few generations it may have been the largest city on earth. It became the place where Greek philosophy, medicine, mathematics, and astronomy were translated and reworked in Arabic, where some of the Islamic world's great early paper workshops fueled an explosion of books, and where the word 'algorithm' was born from a scholar's name. Then, in 1258, it was destroyed. The story of Baghdad is the story of how a city can hold the center of a civilization, and how quickly that center can move.