Let us jump to 647 A.H., when As-Saalih Ayyoob became fatally ill. He was suffering from tuberculosis, which, in addition to his decrepitude, made him bedridden in Cairo. Meanwhile, King Louis IX, of France, wanted to avail himself of the Tatarian invasion of the Eastern part of the Islamic world, and invade the Islamic world from the direction of Egypt and Shaam. We have previously mentioned that Louis IX made an unproductive attempt, in seeking the aid of the Khagan of Tatars, Guyog, son of Ogetai. But even, Louis insisted on carrying out his campaign.
Louis chose Damietta to start his campaign, for at that time it was the most important seaport on the Eastern part of the Mediterranean basin. Thus, what is known in the history as the 7th Crusade began.

We will not discuss the details of this campaign, even though it was of great significance, and its battles were of great importance in Islamic history. But it is irrelevant to talk about this in detail; and they will be discussed thoroughly, Allaah willing, in the book about the Crusades.
King Louis IX descended with his army in Damietta on 20 Safar 647 A.H., and unfortunately, the defending garrison thought that the ill king As-Saalih had died, hence, it unjustifiably withdrew, and Damietta fell easily into the hands of the Crusaders, given that it was the same city which had caused much trouble to the 5th Crusade.

Having been informed about that, King As-Saalih Ayyoob, May Allaah have mercy upon him, grieved, and punished those who were in charge of the fall of Damietta, and expected that the Christian Crusaders would head towards Cairo across the Nile to invade the Egyptian capital, and overthrow the entire state.
For this reason, he took his wise decision to meet them on the way between Damietta and Cairo. He chose Mansoura for it lies on the Nile, and the Crusaders would, inevitably, seize the opportunity to go on board across the Nile, utilizing their large number of ships.
King As-Saalih, May Allaah have mercy upon him, commanded that he should be transported to Mansoura, which lies on the branch of Nile coming from Damietta, to lie in wait for the Crusader army, and get ready for a decisive battle there.
King As-Saalih, May Allaah have mercy upon him, was transported to Mansoura, in spite of his severe illness, and both Faaris Ad-Deen Aqtai and Rukn Ad-Deen Beebars started putting the war plan to meet the Christians in Mansoura.

http://300035.dotdothktrade.tech/en/artical/20774/campaign-louis-IX