Harezmshah State had shown great heroism against the invasion of the Mongolians, but it could not stop their advance. Their ruler, Celaleddin Mengubirti had withdrawn towards the west and he had found a state with the capital city of Meraga that was located in Georgia, the Western Iran, the Caucasus and the Eastern Anatolia and he tried to prevent the torrent invasion of Mongolians. Therefore, there were friendly relationship between Celaleddin who was neighbouring to the Anatolian Seljuks State and Alaeddin Keykubad. However, when Celaleddin Mengubirti invaded and destroyed the city of Ahlat that was one of the most important centres of the Turk-Islam culture, this friendly relationship was on strained terms. Meanwhile, the intervention of the Abbasi caliph and the attempts of the Eyyübi state in order to put the relationship in order were of no use. As a result, Alaeddin Keykubad advanced against Celaleddin Mengubirti with a great army. The two armies encountered in the place of Yassicimen within the environs of Erzincan. In this cruel war that took place on the date o 10th August 1230, Celaleddin Mengubirti was defeated. Then, Sultan captured Erzurum. Pursuant to the death of Celaledin in the year of 1231, the Mongolians had become the neighbours of the Anatolian Seljuks State. The Mongolian raids of despoil extended towards Malatya. Alaeddin Keykubad who had perceived of the Mongolian threat before pretended to have accepted the heavy conditions of Ogedey in order to keep his country away from this danger. Nevertheless, he did not refrain from taking proper precautions. He fortified the castles along the borders and Harezm military units that had been in the service of Seljuk state pursuant to the death of Celaleddin were settled in these border castles.