 On 17 April , General Abrams requested that the president approve Operation <unk> , covert tactical airstrikes in support of Studies and Observations Group <unk> elements " across the fence " in Cambodia . This authorization was given , allowing U.S. aircraft to penetrate 13 miles ( 21 km ) into northeastern Cambodia . This boundary was extended to 29 miles ( 47 km ) along the entire frontier on 25 April . <unk> was terminated on 18 May after 156 sorties had been flown . The last Menu mission was flown on 26 May . 
