Möngke Khan commenced a military campaign against the Chinese Song dynasty in southern China. The Mongol force that invaded southern China was far greater than the force they sent to invade the Middle East in 1256. He died in 1259 without a successor. Kublai returned from fighting the Song in 1260 when he learned that his brother, Ariq Böke, was challenging his claim to the throne. Kublai convened a kurultai in Kaiping that elected him Great Khan. A rival kurultai in Mongolia proclaimed Ariq Böke Great Khan, beginning a civil war. Kublai depended on the cooperation of his Chinese subjects to ensure that his army received ample resources. He bolstered his popularity among his subjects by modeling his government on the bureaucracy of traditional Chinese dynasties and adopting the Chinese era name of Zhongtong. Ariq Böke was hampered by inadequate supplies and surrendered in 1264. All of the three western khanates (Golden Horde, Chagatai Khanate and Ilkhanate) became functionally autonomous, although only the Ilkhans truly recognized Kublai as Great Khan. Civil strife had permanently divided the Mongol Empire.