[1] He started World War II as commander of a Gebirgsjäger-Regiment and fought in Poland, Norway, Yugoslavia, and the Soviet Union, where his unit advanced into the Caucasus.

On 17 December 1942, he replaced Hubert Lanz as commander of the 1st Mountain Division, when the Division was retreating to the Kuban Bridgehead, and in April 1943 he was awarded the Knight's Cross of the Iron Cross for his contribution to the defence of the Bridgehead.

Still under Stettner's command, the division committed several war crimes in Yugoslavia and Greece, including the Massacres of Kommeno, Mousiotitsa, Lyngiades, Borovë and the Massacre of the Italian Acqui Division.

He was described as a "small, meticulous man, who suffered a complex of inferiority and was driven by a tremendous sense of pride that led him to keep political opinions to himself.

This way of treating civilians in Epirus led to frequent frictions with his immediate superior, the devout Catholic General Hubert Lanz".