This book rewrites the history of Christian peace ethics. Christian reflection on reducing violence or overcoming war has roots extending back to ancient Roman philosophy, and it eventually decisively influenced the formation of modern international law. This study traces the development of the tradition from Cicero, Augustine and Thomas Aquinas to early modern thinkers including Vitoria, Suarez, Martin Luther, Hugo Grotius and Immanuel Kant. These sources influenced modern peace ethics’ cosmopolitanism and international law-based approach, as can be found in the late Pope John Paul II’s peace teaching.