The German army's first campaign in the far north was an outstanding success: between April and June 1940 German forces of less than 20,000 seized Norway, a state of three million people, for minimal losses. The army learned new skills to fight effectively in snow and ice. As the terrain prohibited the use of tanks and heavy artillery, and lack of airfields restricted the employment of aircraft, the war became an infantry duel, waged across a frozen landscape. The war in the far north was a most effective campaign, and yet, despite the losses inflicted on the Red Army and Allied convoys, the Wehrmacht resources committed there ultimately drained the German war effort. In the end, Hitler's obsession with the prospect of an Allied invasion of Norway diverted men and materiel from the main effort of the Allies.