 Complete the following phrase:  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 