Baghdad had for centuries been the capital of the Abbasid Caliphate, the third caliphate and whose rulers were descendants of Abbas, uncle of Muhammad. At the city's peak, it was populated by approximately one million people and was defended by an army of 60,000 soldiers. The Caliphs of the 12th and 13th centuries had begun to ally with the expanding Mongol Empire in the east. It has been rumored that some Crusader captives may have been sent as tribute to the Mongol khagan. By 1241 Abbasids had adopted the practice of sending annual tribute to the court of the khagan.

In 1257, Möngke resolved to establish firm authority over Mesopotamia, Syria, and Iran. The khagan gave his brother, Hulagu, authority over a subordinate khanate and army, the Ilkhanate, and instructions to compel the submission of various Muslim states, including the caliphate. Though not seeking the overthrow of Al-Musta'sim, Möngke ordered Hulagu to destroy Baghdad if the Caliph refused his demands of personal submission to Hulagu and the payment of tribute in the form of a military detachment, which would reinforce Hulagu's army during its campaigns against Iranian Ismaili states.

On January 29, the Mongol army began its siege of Baghdad, constructing a palisade and a ditch around the city. Employing siege engines and catapults, the Mongols attempted to breach the city's walls, and, by February 5, had seized a significant portion of the defenses. Realizing that his forces had little chance of retaking the walls, Al-Musta'sim attempted to open negotiations with Hulagu, who rebuffed the Caliph. Five days later, on February 10, the city surrendered.