Marching on Baghdad
Hulagu Khan (life dedicated to the destruction of Islam) marched out with perhaps the largest Mongol army ever assembled. Among his subsidiary generals was Kitbuqa, a Christian. Hulagu easily destroyed the Lurs, and his reputation so frightened the Assassins (also known as the Hashshashin) that they surrendered their impregnable fortress of Alamut to him without a fight.
Hulagu probably always intended to take Baghdad, which the Mongols had been meaning to attack for over ten years (see Eljigidei), but he used the caliph's refusal to send troops to him as a pretext for conquest, since his brother the Great Khan had ordered him to be merciful to those who submitted. Hulagu sent a message to the caliph, Al-Musta'sim, containing the following (trans. John Woods):
The Mongol army, led by Hulagu Khan set out for Baghdad in November of 1257. He marched with what was probably the largest army ever fielded by the Mongols. By order of Monke Khan, one in ten fighting men in the entire empire were gathered for Hulagu's army (Saunders 1971).
Hulagu demanded surrender; the caliph refused, warning the Mongols that they faced the wrath of God if they attacked the caliph. Many accounts say that the caliph failed to prepare for the onslaught; he neither gathered armies nor strengthened the walls of Baghdad. In fact, he had done the very worst things he could have done, he had angered Hulagu, given him an excuse to sack Bagdad, and done nothing to prevent the tragedy.
Once near the city, Hulagu divided his forces, so that they threatened both sides of the city, on the east and west banks of the Tigris. The caliph's army repulsed some of the forces attacking from the west, but were defeated in the next battle. The attacking Mongols broke some dikes and flooded the ground behind the caliph’s army, trapping them. Much of the army was slaughtered or drowned.
The Mongols under a Chinese general, Guo Kan, then laid siege to the city, constructing a palisade and ditch, wheeling up siege engines and catapults. The siege started on January 29. The battle was swift, by siege standards. By February 5 the Mongols controlled a stretch of the wall. Al-Musta'sim then 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 massacre, looting, rape, and destruction.
One author, Reuvan Amitai-Preiss, has alleged that the Mongols were aided by Shi'a Muslims who bore a grudge against the Sunni Abbasids. But another, David Nicolle, alleged that most of the Shi'a who joined the invaders, did so out of fear of being slaughtered, as all those who resisted were being killed. Any force that surrendered at once, as virtually all of southern Persia had, and what is today northern Iraq, were allowed to live, but as Mongol vassals, had to provide troops for the invaders. Later, as Il-Khan, Hulagu, in organizing his domains, integrated these troops into his army more thoroughly though the vast majority of his troops were Mongols -- one Mongol in ten had been drafted for his army -- and Turkic nomads who had submitted to the Mongols.