After a Tibetan uprising against China in 1959, the political and religious head of Tibet, the Dalai Lama, fled to India and was granted political asylum.
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This was resented by China.
Chinese intrusions into India followed, and in 1962, the Indian forces in NEFA were defeated in a surprise Chinese attack. China, however, announced a ceasefire and withdrew behind the Line of Actual Control (LAC) as it existed in 1959. The defeat exposed the weaknesses of India’s external security.
For some time after 1962, Sino-Indian relations remained strained. China supported Pakistan in the Indo-Pak wars of 1965 and 1971, and supplied military and nuclear technology to Pakistan. China also resented India’s nuclear test in 1974 and the absorption of Sikkim into India in 1975.