This book assembles and assesses for the first time the ethics of war and peace in the writings of Paul Tillich. It sketches the evolution of Tillich’s thought from the period of his service in the German Imperial Army through the time of the Cold War. The work begins by analyzing Tillich’s theological roots and his World War I chaplaincy sermons as the starting point for his thoughts on power and nationalism. Then, Religious Internationalism looks to his postwar turn to socialist thought and his participation in religious socialism, fueling his cultural analyses and culminating in his forced emigration under Hitler. Next, it probes the American interwar period, giving special attention to Tillich’s self-described boundary perspective as well as the one treatise he wrote on religion and international affairs. The book then examines his Voice of America speeches, written and broadcast into his former homeland during World War II. It next considers Tillich’s message to his English-speaking audience of that period, emphasizing social and world reconstruction. The discussion continues by examining his vision of a path toward personhood in a bipolar world. Finally, the book constructs Tillich’s ethics of war and peace as an ethic of religious internationalism, suggesting adjustments intended to give it more universal significance. The study concludes that Tillich’s thought has provocative contributions to make to debates regarding civilizational conflict, economics and international justice, trade and globalization, the defense of unprotected minorities, and immigration policy.