Hulagu's army greatly expanded the southwestern portion of the Mongol Empire, founding the Ilkhanate of Persia, a precursor to the eventual Safavid dynasty, and then the modern state of Iran. Under Hulagu's leadership, the siege of Baghdad (1258) destroyed Baghdad's standing in the Islamic world and weakened Damascus, causing a shift of Islamic influence to the Mamluk Sultanate in Cairo.
Hulagu was born to Tolui, one of Genghis Khan's sons, and Sorghaghtani Beki, an influential Keraite princess. Sorghaghtani successfully navigated Mongol politics, arranging for all of her sons to become Mongol leaders. She was a Christian of the Church of the East (often referred to as "Nestorianism") and Hulagu was friendly to Christianity. Hulagu's favorite wife, Doquz Khatun, was also a Christian, as was his closest friend and general, Kitbuqa. It is recorded however that he converted to Buddhism as he neared death, against the will of Doquz Khatun. The erection of a Buddhist temple at Ḵoy testifies his interest in that religion.
Hulagu had at least three children: Abaqa Khan, Tekuder, and Taraqai. Abaqa was second Ilkhan of Iran from 1265–82, Teguder Ahmad was third Ilkhan from 1282–84, and Taraqai's son Baydu became Ilkhan in 1295. Mīr-Khvānd mentions two more children, given as Hyaxemet and Tandon in an early translation; Hyaxemet initially served as governor of Armenia and Azerbaijan, while Tandon was given Diyarbakır and Iraq. The order of birth is listed as Abaqa, Hyaxemet, Tandon, Teguder, then Taraqai. His daughter-in-law, Absh Khatun, was sent to Shiraz to reign in 1263.
Hulagu's brother Möngke Khan had been installed as Great Khan in 1251. In 1255, Möngke charged Hulagu with leading a massive Mongol army to conquer or destroy the remaining Muslim states in southwestern Asia. Hulagu's campaign sought the subjugation of the Lurs of southern Iran, the destruction of the Assassins, the submission or destruction of the Abbasid Caliphate in Baghdad, the submission or destruction of the Ayyubid states in Syria based in Damascus, and finally, the submission or destruction of the Bahri Mamluke Sultanate of Egypt. Möngke ordered Hulagu to treat kindly those who submitted and utterly destroy those who did not. Hulagu vigorously carried out the latter part of these instructions.
Hulagu marched out with perhaps the largest Mongol army ever assembled – by order of Möngke, two-tenths of the empire's fighting men were gathered for Hulagu's army. He easily destroyed the Lurs, and the Assassins surrendered their impregnable fortress of Alamut without a fight, accepting a deal that spared the lives of their people.
Hulagu's Mongol army set out for Baghdad in November 1257. Once near the city he divided his forces to threaten the city on both the east and west banks of the Tigris. Hulagu demanded surrender, but the caliph, Al-Musta'sim, refused. Due to Treason of Abu Alquma(An advisor to Al-Muta'sim) an uprising in baghdad Army took place and Siege of Baghdad happened. The attacking Mongols broke dikes and flooded the ground behind the caliph's army, trapping them. Much of the army was slaughtered or drowned.
The Mongols under Chinese general Guo Kan laid siege to the city on January 29, 1258, constructing a palisade and a ditch and wheeling up siege engines and catapults. The battle was short by siege standards. By February 5 the Mongols controlled a stretch of the wall. The caliph tried to negotiate but was refused. On February 10 Baghdad surrendered. The Mongols swept into the city on February 13 and began a week of destruction. 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 quantity of books flung into the river. Citizens attempted to flee but were intercepted by Mongol soldiers.