自衛団
On 1 September 1939, Germany invaded Poland. Peiper joined Himmler on board his special train and, on occasion, acted as his liaison to Adolf Hitler, travelling with Erwin Rommel to accompany Hitler as he met with Wehrmacht generals near the front. Peiper was with Himmler on 20 September in Bydgoszcz (Bromberg) when they witnessed the execution of 20 Poles organized by the leader of the local Volksdeutscher Selbstschutz and SS functionary, Ludolf von Alvensleben. As Peiper later told Ernst Schäfer, Hitler had ordered Himmler to eliminate "Polish intellectuals".