On Friday 1 September 1939, German forces invaded Poland starting World War II in Europe. I. "Gruppe" of ZG 26 was tasked with patrolling the North Sea. In October, the "Gruppe" began equipping with the Messerschmitt Bf 110 C heavy fighter. In December, I. "Gruppe" was moved to Lippstadt. Von Rettberg was appointed "Gruppenkommandeur" (group commander) of II. "Gruppe" of ZG 26 on 1 April 1940. He let this unit during the Battle of France, Battle of Britain and the Invasion of Yugoslavia.