Upon the outbreak of the Second World War, in September 1939, Kappler was serving as an SS-Hauptsturmführer (captain) out of the Stuttgart Gestapo main office.
Following the armistice between Italy and the Allied Forces on 8 September 1943, the German military occupied Rome and Kappler was ordered to serve as Chief of the Security Police and Security Service (Oberbefehlshaber des Sicherheitspolizei und SD) for all SS and police units deployed in Rome.