A good essay by theologian-historian Gary Dorrien at “The Immanent Frame”:

today Niebuhr is back in public discussion because he symbolizes, notably to Barack Obama, the possibility of a progressive realism that defends America’s interests more prudently and advances the cause of social justice. Niebuhr, like Obama, blends liberal internationalist and realist motifs, contending that multilateral cooperation is compatible with the power-seeking clash of nations. The case for a strong international community has a realistic basis, that the benefits of cooperation outweigh the costs and risks of not working together. All parties are better off when the most powerful nations agree not to do everything that is in their power and nations work together to create new forms of collective security.

Dorrien emphasizes that Niebuhr’s views changed over the course of his life:

The early Niebuhr played up the irrelevance of Jesus’ love of perfectionism to politics, stressing that Jesus never talked about the realistic limits or consequences of social ethical choices. The later Niebuhr realized that the love ethic kept him and many others in the struggle, whether or not they succeeded. That was its political relevance. Justice could not be defined abstractly; it was a relational term that depended on the motive force of love. The meaning of justice could be determined only in the interaction of love and situation, through the mediation of Niebuhr’s three principles of justice—freedom, equality, and order.

He also makes the important point that “realism” unleavened by moral idealism quickly becomes corrupt. Too often Niebuhr’s thought has been understood as divorcing the two.