The Mongolian invasion took place pursuant to the suppression of the rebellion of Babai. Mongolians who had invaded and destroyed Erzurum in 1242 attempted to invade Anatolia with a force of 30 thousands of people under the command of Bahcu Noyan in the year of 1243. Seljuk State composed a more crowded army than that of the Mongolians together with the reserve forces that were sent by the submissive countries. This army set off to make a counterattack under the command of Sultan Keyhusrev II in person.
The two armies encountered in the place of Kosedag that was located in the east with a distance of 80 km from Sivas. When the Mongolians perturbed the vanguard forces of Seljuk State with a manoeuvre, the army that was deprived of its old commanders left th battlefield with the sultan as their leader. Sultan ran away towards Antalya. Mongolians came to Sivas and then to Kayseri; the city was surrendered through attacks and it was destroyed and despoiled.