The First Indochina War ended in 1954. From his base near Siem Reap, Thanh organized the Khmer Serei militia, mainly recruited from amongst the Khmer Krom, to fight Sihanouk, who had come to regard Thanh as one of his greatest enemies. In his 1959 "Manifesto" of the Khmer Serei, Thanh charged Sihanouk with allowing the "Communistization" of Cambodia at the hands of North Vietnam. The Khmer Serei operated in the border areas of Thailand and South Vietnam, making clandestine anti-Sihanouk radio broadcasts, but made little headway, although they were suggested as a source of military power in a number of coup plots (such as the Bangkok Plot). After the Cambodian military and Lon Nol overthrew Sihanouk in 1970, Thanh was invited to participate in the new Khmer Republic government - initially as an adviser to the Acting Head of State, Cheng Heng - and put his Khmer Serei troops at its service.