Khwarezm was the center of the family of Khawarizm Shaah. It had great multitudes of Muslims, and its forts were among the strongest and the most impenetrable forts the Muslims had at that time. Now, it lies at the borders between Uzbekistan and Turkmenistan, directly on the Amu Darya. For the Muslims, it was of great economic, strategic and political significance.

Due to the great importance of this city, Genghis Khan dispatched his greatest and largest army to it. This army besieged the city for five months, during which it failed to conquer it. They demanded supplies from Genghis Khan who supplied them with a great multitude of soldiers. They then advanced towards the city in succession, and pressed upon it from many sides, until they were able to make a hole in the wall, from which they entered the city, and there was terrible fighting between the Tatars and the Muslims, leaving a great number of casualties on both sides, but the field control was in favor of the Tatars.

Then, new multitudes of the Tatars flowed to the city, and the fatal defeat was the share of the Muslims, who came to suffer from the most severe killing, and started to escape and hide in trenches, tunnels and houses. The Tatars committed one of the most odious crimes, when they ruined a huge dam which was built over the Amu Darya to prevent the flood water from the city. By releasing the water, the city of Khwarezm was entirely drowned.

The water entered into all the tunnels, trenches and houses leaving all the houses of the city ruined by virtue of the tremendous flood and no one was saved from among the inhabitants of the city. That is because whoever was saved from killing, died under the ruins or was drowned. Thus, the city turned into ruins, and the Tatars left it after it had been hidden from the surface of the earth, and replaced with the water of the Amu Darya, leaving no traces of any previous life in it.

In fact, the like of this has never been heard about, save what happened with the people of Nooh (Noah), May Allaah exalt his mention; we seek refuge with Allaah from disappointment after victory; and there is no strength and no power save in Allaah. This bloody event took place also in 617 A.H.

https://en.islamstory.com/index.php/en/artical/20721/invasion-khwarezm