He crossed the Indus River and entered the territory of Kirman, South of Pakistan, and exceeded it to the South of Persia (Iran), and then started to gather supporters. He concluded an alliance with Sa‘d Ad-Deen ibn Dakla against his brother Ghiyaath Ad-Deen.

Jalaal Ad-Deen then started to invade the territory of Persia from South to North, fighting his brother Ghiyaath Ad-Deen, until he arrived at the West of Iran and became near the Abbasid Caliphate. The old relation between the kingdom of Khwarezm and the Abbasid Caliphate was extremely tense. Jalaal Ad-Deen, feeling himself powerful versus the weak Abbasid Caliphate, waged war against the Abbasid Caliphate. Meanwhile, the Tatarian armies were still lying in the East of Iran. There should be no wonder, since most leaders, at that time, were politically squinted, as we have mentioned earlier. Jalaal Ad-Deen entered Basrah with his army and besieged it for two months. Then, he left it and moved northward to come near Baghdad, the capital of the Abbasid Caliphate. Fearing for himself, An-Naasir Li Deen Allaah, the Abbasid Caliph, fortified the city and prepared the forces to repel Jalaal Ad-Deen. Furthermore, he did something odious and disgusting when he communicated with the Tatars, seeking their aid in war against Jalaal Ad-Deen.
Exalted be Allaah!

How could he bring the Tatars to fight Jalaal Ad-Deen, despite the fact that he knew well their long history and wars with the Muslims, even though injustice was in the side of Jalaal Ad-Deen, and the right was in the side of the Caliph? How could he bring the Tatars to relieve him? Did he not know that once the Tatars exterminate Jalaal Ad-Deen, their next step would be to destroy the Abbasid Caliphate?
What did you want to do, O caliph of the Muslims?

You just wanted to prolong the duration of your sovereignty for a few years. You liked to die as a slave of the Tatars instead of being a slave of Jalaal Ad-Deen.
Of course, this argument should not be understood to be in defense of Jalaal Ad-Deen. He was extremely blameworthy for dispersing the power of Muslims, and causing them to taste mutual vengeance, each from the other. How similar was he to Saddaam Husayn! Just as Jalaal Ad-Deen left the fires of the Tatars devour the houses of the Muslims, and engaged himself in fighting with the Abbasid Caliphate, Saddam did the same at the time when the West and the East aspired to the Muslim Ummah in Palestine, Afghanistan, Chechnya and Kashmir. He turned away from all of this, and rather devoted himself to the invasion of Kuwait.

Regardless of such moral and political degradation, the solution is not to seek the aid of disbelieving horrifying and dreadful forces to plant them in the countries of the Muslims to solve their problems and cure their diseases.

An-Naasir Li Deenillaah, the Abbasid Caliph, was like the one who sought shelter in the fire to relieve himself of the scorching heat of sun. He was like the one who was attacked by a young thief in his house, thereupon he hastened to seek the aid of the strongest and greatest thief in the region, who came and removed the weak thief, but, at the same time, he stole the house by himself, and the neighboring houses as well; and there is no strength and no power save in Allaah, The Most High, The Most Great.

Although the caliph sought the aid of the Tatars, they were engaged in spreading their control over the vast areas they had previously occupied, and it was not before the end of 628 A.H., that war broke up between them and Jalaal Ad-Deen. Jalaal Ad-Deen invested that respite to spread his control over the areas surrounding Baghdad, North of Iraq, and North of Persia, and then started to enter into the region of Azerbaijan and its surrounding Islamic territories.
His wars were ferocious and detrimental, although the afflicted were the Islamic states. He committed all kinds of atrocities against their inhabitants, of killing, capturing, robbery and demolition. He seemed to have learnt, from his wars with the Tatars, how to be hard-hearted instead of learning how to be merciful towards those who had previously been tormented at the hands of the Tatars.

Then, Jalaal Ad-Deen spread his control over the Christian kingdom of Kurg, after defeating them bitterly. He made a temporary peace treaty with his brother Ghiyaath Ad-Deen, and joined him to his army, even though each remained cautious of the other.

In this way, the authority of Jalaal Ad-Deen extended from the South of Persia to the North-West of the Caspian Sea. As vast as this territory might be, it was filled with commotions and troubles, not to mention the hostilities Jalaal Ad-Deen left in the hearts of all the governors of the surrounding territories, including An-Naasir Li Deen Allaah, the Abbasid Caliph. It was the policy based on hostilities, troubles and conspiracies which Jalaal Ad-Deen inherited from his father Muhammad ibn Khawarizm Shaah, to which we have previously referred, and it returned with nothing but perdition upon the Ummah. Would that the Muslims understand!

At the end of 622 A.H., the wrongful corrupt caliph, An-Naasir Li Deen Allaah, died, after 47 years of ruling, during which he adopted a bad conduct towards his people. He ruined Iraq, wronged its inhabitants, usurped their property and possessions, made the measures less than what is due, imposed unfair taxes and wrongful judgments. Above all of this, he committed the gravest sin in comparison with which all his sins were considered insignificant; that was to communicate with the Tatars in attempt to cooperate with them against the Muslims.

https://en.islamstory.com/index.php/en/artical/20737/what-did-Jalaal Ad-Deen-do