This is the first book in international relations theory entirely devoted to the political thought of Reinhold Niebuhr. Focusing on the existential theology which lies at the basis of Reinhold Niebuhr's theory of international politics, it highlights the ways in which Niebuhrian realism was not only profoundly theological, but also constituted a powerful existentialist reconfiguration of the Realist tradition going back to Saint Augustine. Guilherme Marques Pedro offers an innovative account of Reinhold Niebuhr's eclectic thought, branching out into politics, ethics, history, society and religion and laying out a conceptual framework through which his work, as much as the realist tradition of international political thought as a whole, can be read. The book calls for the need to revisit classic thinkers within IR theory with an eye to their interdisciplinary background and as a way to remind ourselves of the issues that were at stake within the field as it was growing in autonomy and diversity - issues which remain, regardless of its disciplinary development, at the core of IR's concerns. This book offers an important contribution to IR scholarship, revealing the great historical wealth, intellectual originality but also the limitations and paradoxes of one of the greatest American political thinkers of the twentieth century.