 South Vietnamese forces had already crossed the border on 30 April , launching Operation Toan Thang 42 . 12 ARVN battalions of approximately 8 @,@ 700 troops ( two armored cavalry squadrons from III Corps and two from the 25th Division and 5th Infantry Divisions , an infantry regiment from the 25th Infantry Division , and three Ranger battalions and an attached ARVN Armored Cavalry <unk> from the 3rd Ranger Group ) crossed into the Parrot 's Beak region of Svay Rieng Province . The offensive was under the command of Lieutenant General <unk> Cao <unk> , the commander of III Corps , who had a reputation as one of the most aggressive and competent ARVN generals . During their first two days in Cambodia , ARVN units had several sharp encounters with PAVN forces . The North Vietnamese , <unk> by previous ARVN incursions , however , conducted only delaying actions in order to allow the bulk of their forces to escape to the west . 
