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The reason this happened was one of our top advisors was an Nort Vietnames agent, in fact a general in the North Vietnanese army. He gave away the operations and landing sites to the North ahead of time.
He was only one source. There were others under suspiscion,such as a E-4 clerk who worked in Saigon,and even people who worked on the Senate Foreign Relations committee and on Kissinger's "peace"(puke,gag) team in Paris. PLEASE note that Bubba-1 worked for the Senate Foreign Relations Committee as a summer intern before he went to England and Russia. Re-read his letters to ROTC Col Holmes,and note the part where he says "I was already oppossed to the war even before I worked for the Foreign Relations Committee,and my work there exposed me to secret information most Americans weren't aware of that......". Fullbright was the one who got Bubba-1 the job there (and his scholarship at Oxford),he was the Senator who was the chairman of the committee,and he was also the one whose office was suspected of leaking this information to the enemy.
BTW,for anybody interested about this subject,get the paperback book "Operation Brightlight",and read about Ambassador Sullivan in Laos denying the US military permission to go into Laos to rescue shot down pilots and POW's who were KNEW where they were. You can also read about the suspiscions about other traitors such as the Saigon clerk,and the efforts US forces made to rescue POW's and shot down pilots,DESPITE all the bureaucratic efforts made to stop them.