 On 11 October 1954 , the border was closed by the International Control Commission , after a period of 300 days during which free passage between both halves of Vietnam had been allowed . Under the Geneva Accords , anti @-@ communist military personnel were obliged to evacuate to the south , while communist forces were to be moved north . Civilians were free to move to whichever zone they preferred . During the 300 days , Diệm and U.S. CIA adviser Colonel Edward Lansdale staged a campaign to convince people to move to South Vietnam . The campaign was particularly focused on Vietnam 's Catholics , who were to provide Diệm 's power base in his later years , with the use of the slogan " God has gone south " . Between 800 @,@ 000 and 1 @,@ 000 @,@ 000 people migrated to the south , mostly Catholics . At the start of 1955 , French Indochina was dissolved , leaving Diệm in temporary control of the south . 
