Historically, both Siliguri and Chumbi Valley were part of a highway of trade between India and Tibet. In the 19th century, the British Indian government sought to open up the route to British trade, leading to their suzerainty over Sikkim with its strategic Nathu La and Jelep La passes into the Chumbi Valley. Following the Anglo-Chinese treaty of 1890 and Younghusband expedition, the British established trading posts at Yatung and Lhasa, along with military detachments to protect them. These trade relations continued till 1959, when the Chinese government terminated them. The Doklam area, however, had little role in these arrangements because the main trade routes were either through the Sikkim passes or through the interior of Bhutan entering the Chumbi Valley in the north near Phari. There is some fragmentary evidence of trade through the Amo Chu valley, but the valley is said to have been narrow with rocky faces with a torrential flow of the river, not conducive for a trade route.