About the middle of the seventeenth century, preaching of Christianity was banned in Vietnam. Despite the proscription, Catholic missionaries continued their evangelization of Vietnam. Under the Nguyen dynasty, especially under Kings Minh-Mang, Thieu Tri, and Tu Duc, the Christians were persecuted and labeled "perverse to the public order." Using the persecution of Christians as a pretext, the French conquered Vietnam in the second half of the nineteenth century. Under the French administration, the Catholics enjoyed the support of the government. It was during the Ngo Dinh Diem regime that the Catholics filled key positions in the government, the army, and the police. Today there are about three million Christians in Vietnam, most of them Catholics. Although they represented a small percentage of the population, the Catholics played an important role in the political life of Vietnam during the last three decades prior to the fall of Saigon in 1975.