My dissertation is an analysis of Reinhold Niebuhr’s theological, ethical, and political writings addressing international political relations. It argues that the natural trajectory of Niebuhr’s thought (his “Christian Realism”) when applied to international theory places him within what, in IR study, is called the English School of International Relations theory. The English School is, in most ways, the successor to classical realism which was pioneered by Niebuhr, Hans Morgenthau and others, and was the predominant school of IR theory from approximately the publication of Moral Man and Immoral Society in 1932 until soon after Niebuhr’s death in 1971. By that time, American IR completed its conversion from a humanistic, historical, and ethical study of international politics into a value-free social science on the model of economics. This came to be called “neorealism” or “structural realism,” in part to distinguish it from what then came to be called “classical realism.” Significantly, neorealism abandoned ethical inquiry as a serious component of IR study, marking a sharp departure from classical realism. It also set aside the role of human nature, which was of particular concern to Niebuhr and others in classical realism. Based on Niebuhr’s thought and that of the English School, I focus on the role of ethical reasoning and judgment in international politics and argue for its continued importance. More specifically, I am concerned with (a) the role that Niebuhr assigned to ethical judgment in international politics; (b) how Niebuhr’s notions of moral judgment in IR arose from his Christian and Biblical theology, and the significance of the fact that this is not shared by contemporary classical realists, English School thinkers, or others; and (c) why the English School, while different from Christian Realism, is its logical ally in interpreting international political developments.