 The North Vietnamese had ample notice of the impending attack . A 17 March directive from the headquarters of the B @-@ 3 Front , captured during the incursion , ordered PAVN / NLF forces to " break away and avoid shooting back ... Our purpose is to conserve forces as much as we can " . The only surprised party amongst the participants in the incursion seemed to be Lon Nol , who had been informed by neither Washington nor Saigon concerning the impending invasion of his country . He only discovered the fact after a telephone conversation with the head of the U.S. mission , who had found out about it himself from a radio broadcast . 
