Hulegu Khan, also known as Hülegü or Hulagu (c. 1217 – 8 February 1265), was a Mongol ruler who conquered much of Western Asia. As a son of Tolui and the Keraite princess Sorghaghtani Beki, he was a grandson of Genghis Khan and brother of Ariq Böke, Möngke Khan, and Kublai Khan.

Hulegu's army greatly expanded the southwestern portion of the Mongol Empire, founding the Ilkhanate in Persia. Under Hulegu's leadership, the Mongols sacked and destroyed Baghdad, ending the Islamic Golden Age and the Abbasid dynasty. They also weakened Damascus, causing a shift of Islamic influence to the Mamluk Sultanate in Cairo.

Hulegu was born to Tolui, one of Genghis Khan's sons, and Sorghaghtani Beki, an influential Keraite princess and a niece of Toghrul in 1217. Not much is known of Hulegu's childhood except of an anecdote given in Jami' al-Tawarikh and he once met his grandfather Genghis Khan with Kublai in 1224.

Hulegu's brother Möngke Khan had been installed as Great Khan in 1251. Möngke charged Hulegu with leading a massive Mongol army to conquer or destroy the remaining Muslim states in southwestern Asia. Hulegu's campaign sought the subjugation of the Lurs of southern Iran, the destruction of the Nizari Ismaili state (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 Hulegu to treat kindly those who submitted and utterly destroy those who did not. Hulegu vigorously carried out the latter part of these instructions.

Hulegu 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 Hulegu's army in 1253. He arrived at Transoxiana in 1255. 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 in early 1256. He chose Azerbaijan as his power base, while ordering Baiju to retreat to Anatolia. From at least 1257 onwards, Muslims and Christians of every major religious variety in Europe, the Middle East, and mainland Asia were a part of Hulegu's army.

Hulegu'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. Hulegu demanded surrender, but the caliph, Al-Musta'sim, refused. Due to the treason of Abu Alquma, an advisor to Al-Musta'sim, an uprising in the Baghdad army took place and Siege of Baghdad began. 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 29 January 1258, constructing a palisade and a ditch and wheeling up siege engines and catapults. The battle was short by siege standards. By 5 February the Mongols controlled a stretch of the wall. The caliph tried to negotiate but was refused. On 10 February Baghdad surrendered. The Mongols swept into the city on 13 February and began a week of destruction. The Grand Library of Baghdad (also called 'Bayt al-Hikmah), containing countless precious historical documents and books on subjects ranging from medicine to astronomy, was destroyed. Citizens attempted to flee but were intercepted by Mongol soldiers.

Death counts vary widely and cannot be easily substantiated: A low estimate is about 90,000 dead; higher estimates range from 200,000 to a million. The Mongols looted and then destroyed buildings. Mosques, palaces, libraries, 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. Il Milione, a book on the travels of Venetian merchant Marco Polo, states that Hulegu starved the caliph to death, but there is no corroborating evidence for that. Most historians believe the Mongol and Muslim accounts that the caliph was rolled up in a rug and the Mongols rode their horses over him, as they believed that the earth would be offended if touched by royal blood. All but one of his sons were killed. Baghdad underwent a severe decline in importance after the siege, although according to historian Michal Biran, Hulegu ordered the city rebuilt and the libraries were reopened within two years. Smaller states in the region hastened to reassure Hulegu of their loyalty, and the Mongols turned to Syria in 1259, conquering the Ayyubid dynasty and sending advance patrols as far ahead as Gaza.