Historians argue at length about when the Vietnam War began—or the U.S. role in it—but an excellent claim can be made for 1963. In that year, global media were transfixed by the horrible image of a Buddhist monk burning himself alive in protest against the regime of Ngo Dinh Diem, the president of South Vietnam, who later that same year was assassinated. This crisis, just 50 years ago, set the stage for direct U.S. military intervention.

Of the making of books about Vietnam there is no end, and the response of religious communities in the United States to the war has been amply documented. What gets lost in popular memory is how religious battles within Vietnam itself shaped political attitudes and arguably doomed the anticommunist cause.

Such amnesia is scarcely surprising. In the 1960s, few people paid attention to religion as a political factor. But without understanding the role of religion, and particularly of Catholic Christianity, we miss much of the story of those dreadful years. In its origins, the Vietnam War resembled the later civil-religious strife in Lebanon.