|Mongol Plans for Expansion and Sack of Baghdad|
History reports that Chenghiz Khan initially sought friendship and close relations with the rulers of Iran and Iraq. At first he did not want to invade and attack these lands. To develop friendly relations, he sent his envoy and a trade delegation to the court of the Khwarazm Shah. The pride of Sultan Muhammad Khwarazm Shah (d. 617/1220) caused him to reject the overture and kill the envoy and the traders.
The sultan's arrogance created difficulties for himself and for Iran. It led to a series of battles between the Mongols and the Khwarazmshahis in the eastern parts of Iran?  To be sure the thoughtless action of Khwarazmshah, who perhaps failed to foresee the imminent success of the Mongols, was one of the causes of the Mongol attack. According to Ibn al-'Athir, the sultan regretted his ugly action and was seeking a way out when Chenghiz Khan's message reached him. It said: "You killed my companions and traders and took away their possessions. Now, be ready for a war." 
On the other hand, the Mongols, who at first wanted only to unify the eastern territories, subsequently decided to expand their realm, and by availing the excuse provided by the Khwarazm sultan began their campaigns into Central Asia. Some other factors, too, prompted their attach. In particular, the instigation of the Mongols by the Europeans to attack Islamic lands is notable. During the Crusades such an attack could be a great help to the Christians. In this connection, Armenian Christians were on the side of the Westerners. From the time Chenghiz Khan began to entertain his ambition of expanding his realm into the regions of Western Asia, the Mongols vigorously pursued their conquests, and until his death in 624/1227 many areas of Iran had come under the Mongol onslaughts. These included Rey, Qumm, Kashan and Saveh, that is areas of Iraq al ajam. 
The survival of Jalal al-Din Khwarazm Shah and his attack and retreat tactics in battling the aggressors (until 628/1230) restricted the penetration of the Mongols to the central areas of Iran, and the invaders turned towards the Caucasian areas where they extended their conquests. With the departure from the scene of Jalal al-Din, the Mongols, who had remained content with the occupation of Khorasan, began to invade other areas of Iran as well.
That which is notable is the fact that the Mongols had since then the conquest of Baghdad in their plans. Nearly twenty-one years before the fall of Baghdad, when al-Mustansir bi-Allah was the caliph, the Mongols had launched attacks on the city. This practically shows that individuals like Khwajah Nasir al-Din could not have played a role in instigating the invaders and that the Mongols were already on the move in this direction. Rashid al-Din refers to the Mongol attack of the years 634/1236 and 635/1237, in the following words: History on the Khwajah's Role:
In the beginning of the mentioned period, the Abbasid caliph was al-Mustansir bi-Allah. The Mongol forces under the orders of Baychownian were dispatched for attack and they laid siege to Arbil .... When the Caliph learnt about it, he sent Shams al-Din Arsalan with three thousand cavalrymen. When the Mongols came to know about it, they immediately lifted the siege and retreated. The Caliph asked the jurists to rule as to which of the two, hajj and jihad, should take precedence, and the fatwa was given by consensus in favour of jihad.
Then he ordered that the hajj pilgrimage that year be deferred. He ordered the scholars and the jurists, the elite and the ordinary people of Baghdad to train themselves in archery and the use of arms. He also ordered the ditch and rampart of Baghdad to be built. the Mongols returned another time with the purpose of taking Arbil .... Furthermore, he commanded Amir Arsalan Takin to station himself together with his forces outside Baghdad, awaiting the arrival of the Mongols. When the Mongols came to know about it, they changed their direction towards Daquq and the towns, around Baghdad, killing, pillaging and taking captives .... The Mongols were routed and forced to retreat from Jabal Himrin.
The Turks and the Caliph's slaves pursued them, killing many and liberating the captives of Arbil and Daquq.  Several similar reports pertaining to the attacks in the years 632-635/1234-1237 have been given by other historians.  According to a report by Ibn Al al-Hadid, who himself witnessed the Mongol onslaughts, a Mongol force led by Bajaktai, the Junior, attacked Baghdad on 17 Rabi al-'Awwal 643/1245, which was repulsed and the assailants retreated.  In the year 647/1249 too, the Mongols attacked, this time Khanaqin.  The earlier attack of 643/1245, mentioned by Ibn Abi al-Hadid, is mentioned by Ibn - al-Kazeruni (d. 697/1297)  to have occurred on 17 Rabi al-Thani 642/1244.
There are subsequent evidences which indicate that the Mongols were hostile to the caliph, and they regularly complained about him to the Mongol Khan. This being the case, there was no need for anyone else to incite them against the caliph.
When Mengu (Mongke) (649-658/1251-1260) assumed the leadership of Mongols, Baychownian moved with a vast force to protect his domain in Iran. He sent an emissary to the Khan and had complained about the heretics' (the Ismailis) and the caliph himself. The grand qadi Shams al-Din al-Qazwini, who was at the time near Mengu incited the latter against the Ismailis, mentioning their influence over some regions. 
In the wake of the above developments, Mengu sent his brother Hulagu (Hulegu) to lead expedition in the region, telling him: "...Start from Quhistan (Ismaili possession in east Iran), destroy the walls and forts and then move on to subjugate Iraq. If the caliph is found to be submissive, do not harm him in any way, and if he behaves haughtily and hypocritically send him to join the others."  Khwand Mir reports Mengu as having ordered Hulagu, "...Occupy the lands from the Oxus to the furthest parts of Egypt." 
If we review the behaviour of the caliph before and during the fifty days of confrontation with the Mongol conqueror, it can be seen that he had infuriated Hulagu Khan so much that had the Khawajah or anyone of the nobles or scholars opposed him he too would have been killed as well, as happened in the case of Husam al-Din who had warned Hulagu of dire consequences if the caliph's blood were shed.