Iltis' undergraduate enrollment at the University of Tennessee was interrupted by service in the U.S. Army from 1944 to 1946, initially as a medic. Because of his native proficiency in German, he was transferred to an intelligence unit. After World War II, Iltis was stationed in Germany, where he interrogated captive Wehrmacht and SS officers, including Heinrich Himmler, and processed evidentiary documents to prosecute Nazi war crimes at Nuremberg.