In April 1975, the southern Army of the Republic of Vietnam lost control of Saigon in the Vietnam War, and the city fell to the Vietnam People's Army. Van Hanh University was confiscated by the new communist government, and its buildings turned into dorms. Doan was detained the following year in a mass round-up of intellectuals with United States ties, on the grounds that though he might not be a CIA agent at the moment, he could "become one at any time." On 29 August 1976, he was sent to a re-education camp, where he would be imprisoned without trial for twelve years in a cell shared with 40 other people.