Greene based the titular character played by Audie Murphy on C.I.A. agent Edward Lansdale, who, as head of the Saigon Military Mission, was organizing the Caodaist militias under Trình Minh Thế.
When Mankiewicz was in Saigon scouting locations, he met with Lansdale and struck up a friendship. It was Lansdale who was instrumental in perverting Greene's message in the adaptation of the book. In a letter from to Mankiewicz, Lansdale informs that it was Trình Minh Thế who was responsible for the explosion in front of the Continental Hotel, having obtained the explosives directly from the French. He goes on to write:
...in keeping with your treatment of this actually being a Communist action, I'd suggest that you just go right ahead and let it be finally revealed that the Communists did it after all, even to faking the radio broadcast...
Furthermore, the turning of Greene's message met with approval from the head of the C.I.A. Allen Dulles. He made sure that Greene's book was banned in South Vietnam and that Mankiewicz received all necessary assistance from the US government to film in Saigon.
source: "Edward Lansdale's Cold War"
https://books.google.co.uk/books?id=ntEJR0FuNmoC&pg=PA164&lpg=PA164&dq=ed+lansdale+joseph+mankiewicz&source=bl&ots=5qIE42K4jd&sig=uNBbk-153WhosRpI1py7zd876pg&hl=en&sa=X&ved=0ahUKEwi857zsyfDMAhXEDcAKHRNjBcwQ6AEIKjAD#v=onepage&q=ed%20lansdale%20joseph%20mankiewicz&f=false
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