Zahn was the author of several books and articles, often focusing on the topics of conscience and war. He wrote "Military Chaplains", based on interviews he did with RAF Chaplains who had served in the war. He then wrote "German Catholics and Hitler's Wars", in which he argued priests had aided Hitler by telling Germans it was their religious duty to fight. He later wrote "", about the Austrian conscientious objector who refused to fight in Hitler's army. He was also the co-founder of Pax Christi USA.