Immediately after the Proclamation of Indonesian Independence, the People's Security Army (TKR) was formed and he was appointed Commander of the Palembang Ulu Division, with the rank of Colonel. In a reorganization of TKR forces in Sumatra, Simbolon was appointed Commander of Division I / Lahat (1945–46), who supervised 4 regiments and 15 battalions in South Sumatra. When TKR was changed into the Republic of Indonesia's Army (TRI), Simbolon became Commander of the Garuda VII Division in South Sumatra, which oversaw Lampung, Bengkulu, Palembang, and Jambi.