Franz Lehman served with a military government unit of the U.S. Army during the Second World War and after, beginning as a Private First Class and ending the war as a Corporal. His service took him to the United Kingdom, France, Luxembourg and Germany. After the Allied invasion of Germany in spring 1945, Lehman became part of the military government of Hesse-Palatinate in Neustadt. There he was responsible for finding, compiling and analyzing Nazi party archives in order to aid in the task of Denazification, a legal purge of former Nazis. When the French army took over from the Americans in the region, Lehman asked to be reassigned and joined the History, Political Intelligence, Arts and Monuments Department, responsible for tracking down and preserving both Nazi archives and art and artifacts. During his time in Europe, he undertook a personal mission to send aid to a relative, Aunt Hedi, held at Theresienstadt Concentration Camp in Czechoslovakia, and even attempted to visit her in person there. In 1946, Lehman was demobilized and moved to New York, New York. He maintained correspondence with friends in Germany for years afterward.