He commanded the Gorkhali Army with Sardar Bhakti Thapa and Hasti Dal Shah in 1804 against Garhwal Kingdom due to the Garhwal's discontinuance of annual payments to Kingdom of Nepal. The army succeeded in annexing Garhwal to Nepalese territory extending the territory of Nepal up to the Sutlej river in the west. Bada Amar commanded his troops further to Kangra fort of King Sansar Chand. They rested on Jwalamukhi and ultimately captured the fort. King Sansar Chand aided by 1500 soldiers of Sikh Maharaja Ranjit Singh fought against forces of Amar Singh at Ganesh Valley and retreated back only to attack at the evening. Due to the attack in the evening, Gorkhali Army lost some positions and Bada Amar returned to Sutlej river as per agreement on 24 August 1809 AD. Bada Kaji Amar Singh advised Mukhtiyar (Chief Authority) of Nepal, Bhimsen Thapa, to avoid war with the British because he waged war in person and knew the hardships of war. He was one of the senior Bharadars to have opposed the Anglo-Nepalese War due to prevalence of weak administration in the western front suggesting a possible revolt from the general people of the newly conquered western front.