Reinhold Niebuhr belongs to the greatest American intellectuals of the 20th century. U.S. President Obama called him his »favorite philosopher«. However, Niebuhr's thinking is quite neglected in the German-speaking intellectual sphere. This book intends to fill this lacuna. After an introduction of his Christian Realism it is shown that Niebuhr intensively supported different German socialist resistance groups against the Third Reich inside the United States. Furthermore, his influence on international theory and foreign policy analysis is reconstructed and evaluated in the context of current political developments.