The theologian and philosopher Reinhold Niebuhr rose to prominence in the 1940s and 1950s with his writings and lectures on the struggles between the United States’ liberal democracy and its fascist and totalitarian challengers. In recent years, American political figures on both the right and the left (including President Barack Obama) have rediscovered him. This little book by the late historian Diggins seeks to explain Niebuhr’s continuing appeal. He tracks Niebuhr’s intellectual journey as a critic of Christian idealism and the radical left, a champion of an imperfect social justice, and an advocate of foreign policy realism. Niebuhr grounded his philosophy in a sober view of the limits of reason and collective action and the elusiveness of mankind’s quest for, as he put it, “perpetual peace and brotherhood for human society.” During the Cold War, he fashioned a distinctive realist view of the United States’ mission, urging both activism and restraint, moral conviction and humility. It was a Christian realism in which U.S. foreign policy would seek justice and show mercy, always aware that conflict and misery are inherent in the human condition. Niebuhr’s vision is attractive in part because of the world-weary assumptions on which it is based: no one should expect too much from science, modernity, or the goodwill of others.