This chapter lays out the historical development of Niebuhr’s thought on war and peace in the context of American history and religious thought. It argues that in his early thought he accepts the received wisdom concerning early Christian non-violence, a position that led him to join the “Fellowship of Reconciliation” in 1928. With the Japanese incursions into China in the early 1930s, however, his position began to shift in ways captured in his early exchange with his brother, H. Richard Niebuhr. By the time he delivered the Gifford Lectures, at the very beginning of the Second World War, he has rejected pacifism and begun to develop the positions associated with ‘Christian Realism’. This extended into the early period of nuclear deterrence, though with increasing qualification. By the early 1960s, the perceived lack of restraint led Paul Ramsey to turn to the Catholic just war tradition to articulate a Reformation doctrine of principled love that could clarify which uses of force were acceptable and which had to be rejected. The tradition of Niebuhr persists, however, in such thinkers as John Carlson, whose Christian realist account of war and peace draws directly from Niebuhr and his legacy.