 Complete the following phrase:  During the late 1950s and 60s, Mustang became the centre for Tibetan guerrillas engaged in small operations against the incorporation of Tibet into the Peoples 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 