Treaty of Sinchula is signed whereby Bhutan cedes the areas east of the Teesta River to the British East India Company.

The Duar War was a war fought between British India and Bhutan in 1864 to 1865. It was the only military conflict between the two states since 1774 and resulted in Bhutan losing a fifth of its territory.

Bhutan, officially the Kingdom of Bhutan, is a landlocked country in South Asia, in the Eastern Himalayas between China to the north and northwest and India to the south and southeast. With a population of over 727,145 and a territory of 38,394 square kilometres (14,824 sq mi), Bhutan ranks 133rd in land area and 160th in population. Bhutan is a democratic constitutional monarchy with a King as the head of state and a prime minister as the head of government. The Je Khenpo is the head of the state religion, Vajrayana Buddhism.

Teesta River is a 414 km (257 mi) long river that rises in the Pauhunri Mountain of eastern Himalayas, flows through the Indian states of Sikkim and West Bengal and subsequently enters Bangladesh through Rangpur division. In Bangladesh, it merges with Jamuna River which after meeting some other major rivers of the Bengal delta finally falls into the Bay of Bengal. It drains an area of 12,540 km² (4,840 sq mi). In India, the Teesta flows through Mangan District, Gangtok District, Pakyong District, Kalimpong district, Darjeeling District, Jalpaiguri District, Cooch Behar districts and the cities of Rangpo, Jalpaiguri and Mekhliganj. In Bangladesh, it flows through Lalmonirhat District, Rangpur District, Kurigram District and Gaibandha District. It joins the Jamuna River at Phulchhari Upazila in Bangladesh. 305 km (190 mi) of the river lies in India and 109 km (68 mi) in Bangladesh. The Teesta is the largest river of Sikkim and second largest river of West Bengal after the Ganges.

The East India Company (EIC) was an English, and later British, joint-stock company that was founded in 1600 and dissolved in 1874. It was formed to trade in the Indian Ocean region, initially with the East Indies, and later with East Asia. The company gained control of large parts of the Indian subcontinent and Hong Kong. At its peak, the company was the largest corporation in the world by various measures and had its own armed forces in the form of the company's three presidency armies, totalling about 260,000 soldiers, twice the size of the British Army at certain times.