As of 1827, the entire Assam, Bhutan, Sikkim and Ladakh were part of Tibet and the Sino-Tibetan border were at the Jiang River with most present-day ethnic Tibetan Autonomous Prefectures part of then ethnic Han-Mongol dominated area. Bhutan and Sikkim later gains independence and Sikkim was attacked by the Indian Army in April 1975 and subsequently incorporated into India in May.[81] The entire Assam and Ladakh were merged into the British Raj.