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 its surrender, but the last Abbasid Caliph Al-Musta'sim refused. This angered Hulagu, and, consistent with Mongol strategy of discouraging resistance, he besieged Baghdad, sacked the city and massacred many of the inhabitants. Estimates of the number of dead range from 200,000 to a million.