During the late 1950s and 60s, Mustang became the centre for Tibetan guerrillas engaged in small operations against the incorporation of Tibet into the People's Republic of China in 1959. Tibetan guerrilla group Chushi Gangdruk operated out of Upper Mustang with the intention of raiding PLA positions in Tibet, which led to a border incident that caused the killing of a Nepalese officer who was mistaken as a Tibetan rebel. These guerrillas were aided by CIA and Tibetan Khampas. In the 1970s, after US president Richard Nixon had visited China, CIA withdrew its support and the Nepalese government disbanded Tibetan fighters. In 1961, People's Republic of China and Kingdom of Nepal officially signed a border agreement, setting the border between Mustang and Tibet Autonomous Region set slightly north of the traditional boundary marker demarcated by a stupa at .