 <unk> 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 <unk> , 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 were then forcibly expelled . <unk> , North Vietnamese forces did not oppose the evacuation , though they could easily have done so . 
