 South Vietnamese forces had been rehearsing for just such an operation since late March . On 27 April , an ARVN Ranger Battalion had advanced into <unk> Province to destroy a communist base . Four days later other South Vietnamese troops drove 16 kilometers into Cambodian territory . Lon Nol , who had initially attempted to follow a <unk> policy of his own , requested military aid and assistance from the U.S. government on 14 April . On that day , South Vietnamese forces then conducted the first of three brief cross @-@ border operations under the aegis of Operation Toan Thang ( Complete Victory ) 41 , sending armored cavalry units into regions of Cambodia 's Svay Rieng Province nicknamed the Angel 's Wing and the Crow 's Nest . On 20 April , 2 @,@ 000 South Vietnamese troops advanced into the Parrot 's Beak , killing 144 PAVN troops . On 22 April , Nixon authorized American air support for the South Vietnamese operations . All of these incursions into Cambodian territory were simply reconnaissance missions in preparation for a larger @-@ scale effort being planned by MACV and its ARVN counterparts , subject to authorization by Nixon . 
