It was 1963 and Morris West, Australia’s most successful novelist, was researching his next book. Dismissed by the literati as a middlebrow Graham Greene, the former Christian Brother had won a huge international readership for his religious thrillers – page-turning blockbusters that mixed political and ecclesiastical intrigue with big moral questions and topical world events.

For a Catholic writer with an eye for Catholic subjects, a novel set in South Vietnam must have been an irresistible temptation. Its president, Ngo Dinh Diem, was a pious Catholic of the mandarin class who owed his office to a rigged election, a vicious secret police, wholesale corruption and the military backing of the United States, itself ruled by a Catholic president. His nepotism, despotism and religious bias antagonised the Buddhist majority and fanned the communist insurrection.

West arrived in a country officially dedicated to the Virgin Mary where Buddhist monks were setting themselves alight in protest while the president’s Catholic sister-in-law applauded and called it a barbecue. The Viet Cong were gaining strength and an army coup was brewing, oiled by the CIA. Saigon was a sinister and cynical city, wrote West. He would come to look back on his time there with feelings of guilt and responsibility.

After conversations with a “prelate of episcopal rank” – probably Diem’s older brother, Ngo Dinh Thuc, the archbishop of Hue – West was introduced to Diem himself.

Short and roly-poly with glossy slicked-back hair, Ngo Dinh Diem was a smiling face in a white sharkskin suit. Puffing on a cigarette, he told the author of The Devil’s Advocate: “I want the Americans out.”

West felt like “the man carrying the bomb, but I couldn’t control the explosion”. He found Diem personally impressive but felt obliged, as a citizen of a country about to commit troops to South Vietnam, to report his remarks to the Australian ambassador, who immediately passed them to the Americans. A month later, the military arrested Diem at morning mass, tied his hands behind his back and shot him in the head, an event recorded as ‘accidental suicide’. John F Kennedy had okayed the operation.

West felt guilty that he had somehow contributed to Diem’s assassination. In The Ambassador he thinly fictionalised the events, framing his customary spiritual conflict in the disaster of US policy. By the time the novel appeared in 1965, Australia was conscripting troops for Vietnam. West joined the anti-war movement. It was, he said, “One of the things I’m proudest of in my life.”