At his death in 1227, Genghis Khan divided the Mongol Empire amongst his four sons as appanages, but the Empire remained united under the supreme khan. Jochi was the eldest, but he died six months before Genghis. The westernmost lands occupied by the Mongols, which included what is today southern Russia and Kazakhstan, were given to Jochi's eldest sons, Batu Khan, who eventually became ruler of the Blue Horde, and Orda Khan, who became the leader of the White Horde. In 1235, Batu with the great general Subutai began an invasion westwards, first conquering the Bashkirs and then moving on to Volga Bulgaria in 1236. From there he conquered some of the southern steppes of present-day Ukraine in 1237, forcing many of the local Cumans to retreat westward. The Mongol campaign against the Kypchaks and Cumans had already started under Jochi and Subutai in 1216–1218 when the Merkits took shelter among them. By 1239 a large portion of Cumans were driven out of the Crimean peninsula, and it became one of the appanages of the Mongol Empire. The remnants of the Crimean Cumans survived in the Crimean mountains, and they would, in time, mix with other groups in the Crimea (including Greeks, Goths, and Mongols) to form the Crimean Tatar population. Moving north, Batu began the Mongol invasion of Rus' and spent three years subjugating the principalities of former Kievan Rus', whilst his cousins Möngke, Kadan, and Güyük moved southwards into Alania.