Also of note, the House of Wisdom was destroyed leaving a massive hole in human culture, knowledge and experience.

The Mongols executed Al-Musta'sim and massacred many residents of the city, which was left greatly depopulated. The siege is considered to mark the end of the Islamic Golden Age, during which the caliphs had extended their rule from the Iberian Peninsula to Sindh, and which was also marked by many cultural achievements.

The Mongols were so efficient in their cleansing of the area that the country now called Iran did not reach the same population pre-invasion until the 1920's.

To add to one of my favorite stories from History, read on the origin of Don't kill the Messenger