Tibet was another "Autonomous Area" that China controlled. The Chinese promised autonomy to Tibetans, who followed the religious leader, the Dalai Lama. When China's control over Tibet tightened in the late 1950s, however, the Dalai Lama fled to India. Tibetans responded by rioting.
India welcomed the Dalai Lama and other Tibetans refugees after Tibet's failed revolt in 1959. As a result, resentment between India & China grew. In 1962, when India trespassed across the two countries' unclear border into China's territory, Mao unleashed his forces. China held its border, but resentment continued.