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 the treason of Abu Alquma, an advisor to Al-Muta'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 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The Grand Library of Baghdad, containing countless precious historical documents and books on subjects ranging from medicine to astronomy, was destroyed.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.

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.
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 Hulagu 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 was a depopulated, ruined city for several centuries.
Smaller states in the region hastened to reassure Hulagu of their loyalty, and the Mongols turned to Syria in 1259, conquering the Ayyubid dynasty and sending advance patrols as far ahead as Gaza.

A thousand squads of northern Chinese sappers accompanied the Mongol Khan Hulagu during his conquest of the Middle East.
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 the treason of Abu Alquma, an advisor to Al-Muta'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 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.
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. 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 Hulagu 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 was a depopulated, ruined city for several centuries. Smaller states in the region hastened to reassure Hulagu of their loyalty, and the Mongols turned to Syria in 1259, conquering the Ayyubid dynasty and sending advance patrols as far ahead as Gaza.
A thousand squads of northern Chinese sappers accompanied the Mongol Khan Hulagu during his conquest of the Middle East.
Hulagu Khan