Christianity is, at first glance, not a religion optimally suited to guide foreign policy. With its doctrines of universal love, humility, and turning the other cheek, Christian thinkers since at least Augustine have struggled to reconcile their moral vision with the reality of international life. In this book and others like The Children of Light and the Children of Darkness (1944), Niebuhr played an invaluable role during the tumultuous 1930s and 1940s by bringing Christians to terms with participation in the Second World War and later the Cold War. The Christian doctrine of sinfulness and the Fall meant, according to Niebuhr, the ever-present possibility of evil, which was all too evident in spreading fascist and communist doctrines. Moral action did not imply passivity in the face of sin, nor were leaders of communities bound by the same moral constraints as individuals. Though now primarily remembered for its tough-mindedness, Niebuhr's book bears rereading to remind us that a realistic morality is not the same thing as amoral realism, that power, even in the service of justice, must recognize its own limitations, and that democracies were capable of their own kind of hubris.