In 1828, a delegation of the British East India Company (BEIC) officials on its way to the Nepal-Sikkim border stayed in Darjeeling and decided that the region was a suitable site for a sanatorium for British soldiers. The company negotiated a lease of the area west of the Mahananda River from the Chogyal of Sikkim in 1835. In 1849, the BEIC Superintendent Archibald Campbell and the explorer and botanist Joseph Dalton Hooker were imprisoned in the region by the Sikkim Chogyal. The BEIC sent a force to free them. Continued friction between the BEIC and the Sikkim authorities resulted in the annexation of of territory by the British in 1850. In 1864, the Bhutanese rulers and the British signed the Treaty of Sinchula that ceded the passes leading through the hills and Kalimpong to the British. Further discord between Sikkim and the British resulted in a war, culminating in the signing of a treaty and the annexation by the British of the area east of the Teesta River in 1865. By 1866, Darjeeling district had assumed its current shape and size, covering an area of .