During the reign of Ögedei Khan, the Sultanate of Rum offered friendship and a modest tribute to Chormaqan, a kheshig and one of the Mongols' greatest generals. Under Kaykhusraw II, however, the Mongols began to pressure the sultan to go to Mongolia in person, give hostages and accept a Mongol darughachi.
Under the leadership of Baiju, the Mongol commander, the Mongols attacked Rum in the winter of 1242-1243 and seized the city of Erzurum. Sultan Kaykhusraw II immediately called on his neighbours to contribute troops to resist the invasion. The Empire of Trebizond sent a detachment and the sultan engaged a group of "Frankish" mercenaries. A few Georgian nobles such as Shamadavle of Akhaltsikhe also joined him, but most Georgians were compelled to fight alongside their Mongol overlords.
The decisive battle was fought at Köse Dağ on June 26, 1243. The primary sources do not record the size of the opposing armies but suggest that the Mongols faced a numerically superior force. Baiju brushed aside an apprehensive notice from his Georgian officer regarding the size of the Seljuq army by stating that they counted as nothing the numbers of their enemies: "the more they are, the more glorious it is to win, and the more plunder we shall secure".
Kaykhusraw II rejected the proposal of his experienced commanders to wait for the Mongol attack. Instead, he sent a force of 20,000 men, led by inexperienced commanders, against the Mongol army. The Mongol army, pretending a retreat, turned back, encircled the Seljuq army and defeated it.
When the rest of the Seljuq army witnessed their defeat, many Seljuq commanders and their soldiers, including Kaykhusraw II, started to abandon the battlefield. Eventually, the Seljuq army was left without leaders and most of their soldiers had deserted, without seeing any combat.
After their victory, the Mongols took control of the cities of Sivas and Kayseri. The sultan fled to Antalya but was subsequently forced to make peace with Baiju and pay a substantial tribute to the Mongol Empire.
The defeat resulted in a period of turmoil in Anatolia and led directly to the decline and disintegration of the Seljuq state. The Empire of Trebizond became a vassal state of the Mongol empire. Furthermore, the Armenian Kingdom of Cilicia became a vassal state of the Mongols. Real power over Anatolia was exercised by the Mongols.