Karl Paul Reinhold Niebuhr (June 21, 1892 – June 1, 1971) was an American theologian, ethicist, public intellectual, commentator on politics and public affairs, and professor at Union Theological Seminary for more than 30 years. Also known for authoring the Serenity Prayer, Niebuhr received the Presidential Medal of Freedom in 1964. Among his most influential books are Moral Man and Immoral Society and The Nature and Destiny of Man, the latter of which was written as the result of Niebuhr's delivery of the Gifford Lectures. Niebuhr was also the brother of another prominent theologian and ethicist, H. Richard Niebuhr. Starting as a leftist minister in the 1920s indebted to theological liberalism, he shifted to Neo-Orthodox theology in the 1930s, explaining how the sin of pride created evil in the world, and created the theo-philosophical perspective known as Christian realism. He attacked utopianism as ineffectual for dealing with reality, writing in The Children of Light and the Children of Darkness (1944):
"Man's capacity for justice makes democracy possible; but man's inclination to injustice makes democracy necessary."
Niebuhr's realism deepened after 1945 and led him to support the United States' efforts to confront Soviet communism around the world. A powerful speaker, he was one of the "most influential religious leaders of the 1940s and 1950s in American public affairs. Niebuhr battled with religious liberals over what he called their naïve views of the contradictions of human nature and the optimism of the Social Gospel, and battled with the religious conservatives over what he viewed as their naïve view of scripture and their narrow definition of "true religion". During this time he was viewed by many as the intellectual rival of John Dewey. Niebuhr was also one of the founders of Americans for Democratic Action and spent time at the Institute for Advanced Study in Princeton, New Jersey.
Niebuhr's long-term contributions to political philosophy and political theology involve relating the Christian faith to "realism" in international relations and foreign affairs, away from idealism, and his contribution to modern just war thinking. His work has significantly impacted on international relations theory, with political scientists such as Hans Morgenthau, Kenneth Waltz and Andrew Bacevich noting his influence on their thinking. Numerous politicians and activists such as former President Jimmy Carter, Martin Luther King, Jr., Hillary Clinton, Madeleine Albright, John McCain, and Eliot Spitzer have also noted his influence on their thinking. Historian Arthur M. Schlesinger, Jr. described Niebuhr "the most influential American theologian of the 20th century". and Time Magazine posthumously called Niebuhr "the greatest Protestant theologian in America since Jonathan Edwards". Recent years have seen a renewed interest in Niebuhr's work, in part because of President Barack Obama's stated admiration for Niebuhr.