The Mongol conquest of Persia comprised three Mongol campaigns against Islamic states in the Middle East and Central Asia between 1219 and 1258. These campaigns led to the termination of the Khwarazmian dynasty, the Nizari Ismaili state, and the Abbasid Caliphate of Baghdad, and the establishment of the Mongol Ilkhanate government in their place in Persia.
Genghis Khan had unified the Mongolic peoples and conquered the Western Xia dynasty in the late 12th and early 13th centuries. After a series of diplomatic provocations on the part of Muhammad II, the ruler of the neighbouring Khwarazmian Empire, the Mongols launched an invasion in 1219. The invaders laid waste to the Transoxianan cities of Bukhara, Samarkand, and Gurganj in turn, before obliterating the region of Khorasan, slaughtering the inhabitants of Herat, Nishapur, and Merv, three of the largest cities in the world. Muhammad died destitute on an island in the Caspian Sea. His son and successor, Jalal al-Din, tried to resist the Mongols, but was eventually defeated and forced into exile. Genghis returned to his campaign against the Jin dynasty in 1223, only retaining governance of the northern Khwarazmian regions.
The war had been one of the bloodiest in human history, with total casualties estimated to be between two and fifteen million people. The next three decades saw conflicts of lesser scale but equal destruction in the region. Soon after his accession to the khaganate in 1227, Ögedei Khan sent an army under Chormaqan Noyan to end Jalal al-Din's resistance and subjugate several minor polities in Persia. This was carried out gradually: al-Din was killed in 1231, with Isfahan and Maragheh being besieged and captured the same year; Irbil was captured in 1234; and Georgia was gradually subjugated and vassalised before Chormaqan's death in 1241. Several other Persian towns and cities, such as Hamadan, Ray, and Ardabil, were also captured by the Mongols.
The final stage began in 1254. On the orders of his brother, the Khagan Möngke Khan, Hulagu Khan systematically captured the fortresses of the Nizari Ismaili state in northern Persia, seizing their capital of Alamut in 1256. Hulagu then marched on the Abbasid Caliphate of Baghdad; capturing the city, he ended the 500-year-old Abbasid dynasty by killing the caliph Al-Musta'sim, marking the end of the Islamic Golden Age. Persia would later become the heartland of the Mongol Ilkhanate.
Background
Iran's military and political conditions before the invasion
Expansion of Sultan Muhammad's territory and war with Abbasid Caliph Al-Nasir
After the fall of the Seljuks of Persian Iraq, with the defeat of Toghrul III by Ala al-Din Tekish, the father of Sultan Muhammad, a severe enmity arose over the governance of Western Iran between Abbasid Caliph Al-Nasir and Ala al-Din Tekish. These conflicts continued during the reign of Sultan Muhammad. Therefore, during the reign of Sultan Muhammad, Western Iran became the Battleground of the Khwarazmi troops and the Caliphate troops. In order to destroy Khwarazmian Empire, Al-Nasir not only provoked Ghurid sultans and fanatical religious scholars from Transoxiana against him, but also asked help Nizari Ismaili state, Qara Khitai Empire and Mongol tribes to fight against Khwarazmian Empire. This action not only led to the overthrow of Khwarazmian Empire, but also the overthrow of the Abbasid government.
Simultaneously with the coronation of Sultan Mohammad, the Ghurids captured the remaining lands of Ghaznavid empire and Khorasan cities such as Balkh city after the Overthrow of the Ghaznavids. Sultan Muhammad defeated the Ghurid dynasty in 1212 and captured Ghaznin in 1215, thus he extended the border of his territory to India from the east. In 1209, he conquered Mazandaran, which had been held by the Bavands for a long time. In the 12th century, a group of Khitan people from North China, who were Buddhists, had formed a large government in the province of Kashgar and Hotan called Qara Khitai. In order to block their way, the Khwarazmians had agreed to pay them a yearly tribute. This custom continued until the time of Sultan Muhammad, but he refused to pay tribute to the a "polytheist" king. Sultan Muhammad fought three times with the Qara Khitais. At last In 1210, the Sultan asked help from Kuchlug Khan, a prince of the Naimans and the leader of them, to defeat Qara Khitais. Kuchlug defeated them in Transoxiana and conquer Bukhara and Samarkand, he also managed to usurp the empire from his father-in-law Yelü Zhilugu.