It's a fascinating question. Corso had an admirable military record and the military is very serious about weeding out crackpots. From Wikipedia (
http://en.wikipedia.org/wiki/Philip_J._Corso) :
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In 1945, Corso arranged for the safe passage of 10,000 Jewish WWII refugees out of Rome to Palestine.
During the Korean War (1950-1953), Corso performed Intelligence duties under General Douglas MacArthur as Chief of the Special Projects branch of the Intelligence Division, Far East Command. One of his primary duties was to keep track of enemy prisoner of war (POW) camps in North Korea. Corso was in charge of investigating the estimated number of U.S. and other United Nations POWs held at each camp and their treatment. At later held congressional hearings of the Senate Select Committee on POW/MIA Affairs, Philip Corso would provide testimony that many hundreds of American POW's were abandoned at these camps.[2][3]
Corso was on the staff of President Eisenhower's National Security Council for four years (1953-1957).
In 1961, he became Chief of the Pentagon's Foreign Technology desk in Army Research and Development, working under Lt. Gen. Arthur Trudeau.
When he left military intelligence in 1963, Corso became a key aide to Senator Strom Thurmond.
In 1964, Corso was assigned to Warren Commission member Senator Richard Russell Jr. as an investigator into the assassination of John F. Kennedy.
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This is not the resume of a crackpot. In later years he did nothing to discredit himself or draw attention to himself.
When I look at the man's life I think it speaks for itself as the life of an honest, intelligent man. It also strikes me as realistic that a military man who saw these things could maintain silence his entire life but could not morally die with a secret like that. The debunking attempts were lame, based on the philosophy that this couldn't have happened and therefore didn't (not exactly an impartial scientific view).
I'm pretty sure the book was honest. The guy saw something very strange. Maybe his memory is flawed. Maybe his interpretation of what he saw was wrong. But he saw something.