Mongol invastion was a disaster to the whole known world during 13th century, do you know the famous "Destruction of Baghdad", one of three largest city during medieval, in 1258? it set back the clock of Arab civilization.

"Many historical accounts detailed the cruelties of the Mongol conquerors.

The Grand Library of Baghdad, containing countless precious historical documents and books on subjects ranging from medicine to astronomy, was destroyed. Survivors said that the waters of the Tigris ran black with ink from the enormous quantities of books flung into the river and red from the blood of the scientists and philosophers killed.

Citizens attempted to flee, but were intercepted by Mongol soldiers who killed with abandon. Martin Sicker writes that close to 90,000 people may have died (Sicker 2000, p. 111). Other estimates go much higher. Wassaf claims the loss of life was several hundred thousand. Ian Frazier of The New Yorker says estimates of the death toll have ranged from 200,000 to a million.

The Mongols looted and then destroyed mosques, palaces, libraries, and hospitals. Grand buildings that had been the work of generations were burned to the ground.

The caliph was captured and forced to watch as his citizens were murdered and his treasury plundered. According to most accounts, the caliph was killed by trampling. The Mongols rolled the caliph up in a rug, and rode their horses over him, as they believed that the earth was offended if touched by royal blood. All but one of his sons were killed, and the sole surviving son was sent to Mongolia, where Mongolian historians report he married and fathered children, but played no role in Islam thereafter (see Abbasid: The end of the dynasty).

Hulagu had to move his camp upwind of the city, due to the stench of decay from the ruined city.

Baghdad was a depopulated, ruined city for several centuries and only gradually recovered some of its former glory."