World War II in Europe began on Friday, 1 September 1939, when German forces invaded Poland. Flying with ZG 26, he flew several patrol missions on the Western Front during the Phoney War period. On 9 April 1940, the "Wehrmacht" launched Operation Weserübung, the German assault on Denmark and Norway. Two days later, Strüning was reassigned to the "Zerstörrerstaffel" of "Kampfgeschwader" 30 (KG 30—30th Bomber Wing). Until 9 June, he flew escort missions in support of the German troops at Narvik. For his service in Norway, he was awarded the Iron Cross second Class () on 15 July 1940. On 1 August 1940, Strüning was promoted to "Feldwebel" (sergeant).