The Chagatai Khanate, also known as the Chagatai Ulus, was a Mongol and later Turkicized khanate that comprised the lands ruled by Chagatai Khan, second son of Genghis Khan, and his descendants and successors. With the division of the Mongol Empire after 1259, it became a functionally separate khanate. At its height in the late 13th century, the khanate extended from the Amu Darya south of the Aral Sea to the Altai Mountains on the border of modern-day Mongolia and China, roughly corresponding to the area once ruled by the Qara Khitai.