The Duar War, fought between the British Empire and Bhutan from 1864 to 1865, erupted primarily due to Bhutan's repeated raids into British-held territories like Sikkim and Cooch Behar and the subsequent failure of diplomatic missions, notably one led by Ashley Eden which was met with hostility. In response, the British launched a four-pronged military invasion known as the Duar Field Forces, which, despite initial setbacks and humiliating defeats such as the temporary loss of Dewangiri, eventually captured key forts like Dalimkote and Buxa. The successful British campaign put significant pressure on Bhutan, leading to negotiations and the signing of the Treaty of Sinchula on November 11, 1865. This treaty permanently ceded the Duars region to the British, required the release of all captives, and established peaceful coexistence in exchange for an annual British payment to the Bhutanese government, effectively ending the cross-border raids and solidifying British control in the area.