The realist theology of Reinhold Niebuhr contributed to the disillusion of Christian nationalism and American exceptionalism formerly espoused by progressive Christians. In other words, realism’s theological underpinnings began a logical path of falling dominos that culminated in the rejection of Christian nationalism by 1960s progressive radicals. Significant scholarship has documented the rise of Christian nationalism among conservatives associated with the religious right and Christian America Thesis of the 1980s (Williams, God’s Own Party; Du Mez, Jesus and John Wayne). To understand the rise of conservative expressions of religious nationalism, it is necessary to trace the fall of religious nationalism among progressives. Whereas conservative Christian nationalism attempted to reclaim a Christian America, progressive Christian nationalism aimed at achieving Christian America through a march of progress. Niebuhr bridged the progressive world of idealism in World War I and the radicalism that arose during the Vietnam War. At the beginning of Niebuhr’s career, idealist and progressive Christians touted a triumphant view of “Christian America,” common to the late nineteenth and early twentieth centuries. As he developed his thought amidst the challenges of two world wars and the Cold War, Niebuhr’s thought moved progressive Christianity towards a realistic appraisal of man’s sinfulness and history’s lack of guaranteed success in building the Kingdom of God on earth. This “Niebuhrian turn” set in motion three ideas and their consequences which toppled progressive Christian nationalism. The first domino fell as Niebuhr’s emphasis on the sinfulness of man and society in “Moral Man and Immoral Society” invited self-critique and sober recognition of national pride. These conclusions contributed to another falling domino as Niebuhr’s views of human nature resulted in a realistic appraisal of history and its ironies articulated most clearly in “The Irony of American History.” Lastly, America’s controversial involvement in Vietnam knocked over the final domino, signaling a general end of progressive Christian nationalism and relegating the building of “Christian America” to the right-wing during and after the war. Niebuhr’s warnings against national pride and attempts to master history came to fruition during America’s involvement in Vietnam, and his eventual critique of the war echoed his previous warnings. In other words, the Vietnam War epitomized (for progressives) just about everything Niebuhr spent his life warring against. Vietnam also catalyzed American progressive Christians to express their new Niebuhrian understandings of national and religious identity in contrast to previous wars.