Has there ever been a caliphate?
Although the Chagatai Khanate and the Golden Horde both established themselves in regions already inhabited by Muslims, their invasions of Central Asia and Russia, respectively, did not have the catastrophic effect on the native Islamic faith that the Mongol invasion of Persia and Iraq had. Although the faith prevailed, and the Mongol invaders were eventually converted to Islam, the Mongol destruction of the Islamic heartland marked a major change of direction for the region. By destroying the Islamic empires that existed before they came, the Mongols instigated a new era for the Islamic world, in which most of the region's power would fall to three great empires - the Ottoman, the Safavid, and the Mughal - as we will see in Chapter 5.
The Mongols began their push into Central Asia and Persia in the early 13th century under Genghis Khan. The cities of Bukhara and Samarkand, later to become part of the Chagatai Khanate, fell to Genghis Khan's armies in 1220. From there it was not difficult to raid Persia, and by 1221 the Persian cities of Merv, Nishapur, and Balkh had fallen. In the inevitable pillaging that followed Mongol attacks, the invaders decimated the population of these regions, sparing only the artisans they deemed useful. The Mongols also uprooted many Muslim graves in their wake, including that of Harun al-Rashid, the 8th century Abbasid caliph who was featured in The Thousand and One Nights fables.
The Muslims inflicted their first defeat on the Mongols in 1221 at the Battle of Parwan, in present-day Afghanistan, under the leadership of Jalal al-Din, son of a Central Asian Muslim ruler. The victory provided a temporary morale boost for the Muslim army, but the Mongols soon regrouped and devastated Jalal's troops later that year. After that initial setback, the Mongols swept through Central Asia into Persia and Iraq. The Persian city of Isfahan fell in 1237, and the Mongols gradually moved closer to Baghdad, the centre of the Abbasid caliphate.
The decision to attack the Abbasid caliphate was made at the same time as the election of the Great Khan Mongke in 1251. The Chagatai Khanate and the Golden Horde were already firmly established empires in the Islamic world, and the Great Khan disliked the fact that his new Muslim subjects worshipped a man - the caliph - that they deemed to be in a higher position than the Great Khan. Mongke decided to send his brother, Hulegu, into Iraq at the head of the invading Mongol army, with the goal of sacking Baghdad and destroying the Abbasid caliphate there. Hulegu set out in 1253, and en route he encountered the Muslim group known as the Assassins, an Ismaili sect that practised an extreme version of Shi'ism. The Assassins were based in Alamut, in northwestern Persia, which Hulegu reached in 1255. The Mongols easily destroyed the small Assassins force, and the remaining members of the group fled south to the Sindh region of present-day Pakistan, where they lived as an underground sect for centuries. After this victory, the Mongols had an open path into Baghdad. Great Khan Mongke had instructed Hulegu to attack the Abbasid caliphate only if it refused to surrender to the Mongols. The Abbasids, led by the caliph, Musta'sim, indeed refused to surrender, making a battle inevitable.
Before the fighting even began, the Abbasids were at a disadvantage. While they theoretically had a large enough army to compete with the Mongols, their troops had been neglected by the caliphate and were not prepared for battle at the time of the Mongol invasion. Another problem for the Abbasids was the centuries-old rift between the Sunni and Shi'ite Muslims. The caliphate was Sunni, as were most of its subjects, but there was a Shi'ite minority under Abbasid control who welcomed the Mongol invaders as a potential means of bringing down the Sunni caliph. Many Shi'ites in Iraq joined the Mongol forces for that reason. Additionally, the caliph's vizier, or second-in-command, was himself Shi'ite, and it has been suggested that he might have also co-operated with the Mongols in attacking the caliphate. The Mongols also had the support of non-Muslims under Abbasid control. Many Christians in the region saw the Mongols as saviours, hoping that by decimating Islam's adherents, the faith itself would also be destroyed. Indeed, in return for Christian support, the Mongols - some of whom were Nestorian Christian themselves - spared Christian churches and communities from their pillaging.
With all these factors working against the Abbasids, the fall of Baghdad and the destruction of the caliphate in 1258 came rather quickly. The caliph himself, Musta'sim, was captured and killed, and the 500-year-old Abbasid dynasty came to an abrupt and violent end. With Iraq and Persia thus under Mongol control, Hulegu continued west, towards Syria and Egypt. Ayyubid descendants of Saladin held power in Syria at this time, while the European Crusaders had a tenuous hold on the Syrian coast. Egypt, meanwhile, was still recovering from the coup that had ousted the Ayyubids and brought the Mamluks, a class of Turkish slave soldiers, to power. As professional soldiers, the Mamluks would present the Mongols with their only serious and continuous challenge. Syria, however, was easily defeated, since the Ayyubids and Crusaders refused to join forces in defending their territory. The major cities of Aleppo and Damascus fell within a month of each other in 1260, but an immediate invasion of Egypt was halted by the death of the Great Khan Mongke in Mongolia....
http://www.ucalgary.ca/applied_history/tutor/islam/mongols/ilkhanate.html