By March 1972, Lon Nol and his brother had managed to drive Sirik Matak from power. After Sirik Matak had sacked Keo An, a dissident academic, Lon Non organised a series of vocal student demonstrations against him calling for his removal. Sirik Matak resigned, and (ostensibly for his 'protection') was placed under effective house arrest. Lon Nol used the crisis to oust the Head of State, Cheng Heng, and took over the role himself, appointing the veteran anti-Sihanouk nationalist Son Ngoc Thanh as Prime Minister. Thanh, leader of the Khmer Serei, had recruited FANK reinforcements amongst the Khmer Krom of southern Vietnam, and the loyalty he commanded amongst these comparatively elite, US-trained troops meant that his support for the Republic's fragile government remained essential.