In the 1640s, the last years of the Ming Dynasty, geographer Xu Xiake explored the Salween River country and determined that the upper Salween, Mekong and Red rivers (previously believed to be part of the same river system) were in fact separate. After the rise of the Qing Dynasty China entered a new period of western expansion. In 1717 the Dzungar Khanate conquered Tibet, and China sought to expel them from the region. In 1718 the Qing sent an army but were unsuccessful in reaching Tibet, reaching only as far as the Salween River where they were defeated by Dzungar troops in the Battle of the Salween River. In response, the Qing sent a larger force to Tibet in 1720, driving out the Dzungars and establishing Qing rule of Tibet.