A study of the life and work of Gordon Zahn (1918-2007), one of the most significant and formative figures in the history of the US Catholic peace movement. During World War II, Zahn was one of only a handful of US Catholic conscientious objectors, an experience that first put him in contact with Dorothy Day and the Catholic Worker. Following the war, Zahn completed a PhD in sociology at The Catholic University of America where Paul Hanly Furfey’s "supernatural sociology" had a profound effect on him. After joining the faculty at Loyola University, Chicago, Zahn won a Fulbright year in Germany (1956-57) where he discovered the story of Franz Jägerstätter, the Austrian Catholic farmer, husband, and father who was beheaded bythe Nazis in 1943 for refusing to participate in Hitler’s military. It was Zahn’s 1964 book  In Solitary Witness that introduced Jägerstätter to an international audience and led to his beatification by Pope Benedict XVI in 2007. This work, along with Zahn’s several other books, secured his place as a leading intellectual in the Catholic peace movement. His work influenced the discussions of war and peace at the Second Vatican Council and he was enlisted as an expert witness during the drafting of the US Catholic Bishops’ pastoral letter on nuclear weapons,  The Challenge of Peace (1983). Zahn made one of the most intellectually developed cases in the US for "Catholic pacifism"—two terms that both Catholics and pacifists had long regarded as deeply incompatible. Forging a Catholic pacifist position led him to be far more perceptive and critical of US political and military institutions than most of fellow US Catholics. Ultimately, Zahn saw the Catholic Church as the only institution capable of resisting, and support those who resist, modern warfare and the modern war-making state. His traditional view of the Church grounded his claim that Catholics can be pacifist and hisfurther assertion that the Church should serve as a "source of dissent" for Catholics and others, especially during times of war. While marginalized within the broader US Catholic intellectual community, Zahn found common cause with other Catholic luminaries who are now seen to be ahead of their time, including Dorothy Day, Thomas Merton, and Daniel Berrigan.  Gordon Zahn’s story serves as a template for telling the broader story of the US Catholic peace movement and the development of Catholic attitudes on war and conscientious objection that took place within the twentieth century.