On 27 November 1941, Rainer was appointed as the "Gauleiter" and "Reichsstatthalter" of Reichsgau Carinthia, which also involved ruling over the adjacent occupied Yugoslavian territories in Upper Carniola. On 16 November 1942, Rainer was made Reich Defense Commissioner of Carinthia. On 21 June 1943 he was promoted to "SS–Obergruppenführer". After Italy abandoned its German ally on 8 September 1943, Rainer took over the Operational Zone of the Adriatic Littoral as High Commissioner. This act established him as the chief of the civil administration in the Italian region of Friuli, as well as in Yugoslavian Istria and Inner Carniola.