665 Days In Iraq

In 1257, Hulagu Khan amassed an unusually large army, a significant portion of the Mongol Empire's forces, for the purpose of conquering Baghdad. When they arrived at the Islamic capital, Hulagu Khan demanded surrender but the last Abbasid Caliph Al-Musta'sim refused. This angered Hulagu, and, consistent with Mongol strategy of discouraging resistance, Baghdad was decimated. Estimates of the number of dead range from 200,000 to a million.
The Mongols destroyed the Abbasid Caliphate and The Grand Library of Baghdad (Arabic بيت الحكمة Bayt al-Hikma, lit., House of Wisdom), which contained countless, precious, historical documents. The city has never regained its status as major center of culture and influence. Some historians believe that the Mongol invasion destroyed much of the irrigation infrastructure that had sustained Mesopotamia for many millennia. Other historians point to soil salination as the culprit in the decline in agriculture.
The mid-14th-century Black Death ravaged much of the Islamic world. The best estimate for Middle East—Iraq, Iran, Syria, etc.—is a death rate of a third.
In 1401, warlord of Mongol descent Tamerlane (Timur Lenk) invaded Iraq. After the capture of Bagdad, 20,000 of its citizens were massacred. Timur ordered that every soldier should return with at least two severed human heads to show him (many warriors were so scared they killed prisoners captured earlier in the campaign just to ensure they had heads to present to Timur).