Debate rages within the Catholic Church about the ethics of war and peace, but the simple question of why wars begin is too often neglected. Catholics' assumptions about the causes of conflict are almost always drawn uncritically from international relations theory—a field dominated by liberalism, realism, and Marxism—which is not always consistent with Catholic theology.

In The Origins of War, Matthew A. Shadle examines several sources to better understand why war happens. His retrieval of biblical literature and the teachings of figures from church tradition sets the course for the book. Shadle then explores the growing awareness of historical consciousness within the Catholic tradition—the way beliefs and actions are shaped by time, place, and culture. He examines the work of contemporary Catholic thinkers like Pope John Paul II, Jacques Maritain, John Courtney Murray, Dorothy Day, Brian Hehir, and George Weigel. In the constructive part of the book, Shadle analyzes the movement within international relations theory known as constructivism—which proposes that war is largely governed by a set of socially constructed and cultural influences. Constructivism, Shadle claims, presents a way of interpreting international politics that is highly amenable to a Catholic worldview and can provide a new direction for the Christian vocation of peacemaking.