Thieu then authorized Operation Cuu Long , in which ARVN ground forces , including mechanized and armoured units , drove west and northwest up the eastern side of the Mekong River from 9 May – 1 July . A combined force of 110 Vietnamese Navy and 30 U.S. vessels proceeded up the Mekong to Prey Veng , permitting IV Corps ground forces to move westward to Phnom Penh and to aid ethnic Vietnamese seeking flight to South Vietnam . Those who did not wish to be repatriated was then forcibly expelled . Surprisingly , North Vietnamese forces did not oppose the evacuation , though they could easily have done so .