In 1860 Eden accompanied a force ordered to invade the hill state of Sikkim in the Himalayas, as political agent, and in March 1861 he signed the Treaty of Tumlong with the raja, Sidkeong Namgyal, which secured protection to travellers and free trade. This success caused Eden to be appointed special envoy to the hill state of Bhutan in 1863. He was accompanied by no armed force and his demands were rejected. He signed a treaty favourable to the Bhutiás. This treaty was not ratified by the supreme government, and the Bhutan War resulted.