The Siege of Baghdad, occurring from January 29 until February 10, 1258, entails the investment, capture, and sacking of Baghdad, the capital of the Abbasid Caliphate, by Ilkhanate Mongol forces and allied troops.

The Mongols are under the command of Hulagu Khan, brother of the khagan Möngke Khan, and intend to further extend their rule into Mesopotamia but not to directly overthrow the Caliphate.

Möngke, however, has instructed Hulagu to attack Baghdad if the Caliph Al-Musta'sim refuses Mongol demands for his continued submission to the khagan and the payment of tribute in the form of military support for Mongol forces in Iran.Hulagu had begun his campaign in Iran, with several offensives against Nizari groups, including the Assassins, whose stronghold of Alamut his forces have seized.

He now marches on Baghdad, demanding that Al-Musta'sim accede to the terms imposed by Möngke on the Abbasids.

Although the Abbasids had failed to prepare for the invasion, the Caliph believes that Baghdad cannot fall to invading forces and refuses to surrender.

Hulagu subsequently besieges the city, which surrenders on February 10.

During the next week, the Mongols sack Baghdad, committing numerous atrocities and destroying the Abbasids' vast libraries, including the House of Wisdom.

The Mongols execute Al-Musta'sim and massacre many residents of the city, which is left greatly depopulated.

The siege is considered to mark the end of the Islamic Golden Age, during which the caliphates had extended their rule from the Iberian Peninsula to Sindh, and which was also marked by many cultural achievements.