Germany and Finland had been at war with the Soviet Union (USSR) since Operation Barbarossa began in June 1941, co-operating closely in the Continuation War and Operation Silver Fox with the German 20th Mountain Army () stationed in Lapland. As early as the summer of 1943, the German high command "Oberkommando der Wehrmacht" () began to plan for the eventuality that Finland might negotiate a separate peace agreement with the Soviet Union. The Germans planned to withdraw their forces northward in order to shield the nickel mines near Petsamo (). During the winter of 1943–1944, the Germans improved the roads from northern Norway to northern Finland by extensive use of prisoner-of-war labour in certain areas. Casualties among the labouring prisoners were high, in part because many of them had been captured in southern Europe and were still in summer uniform. In addition, the Germans surveyed defensive positions and planned to evacuate as much materiel as possible from the region, and meticulously prepared for withdrawal. On 9 April 1944, the German withdrawal plan was designated as Operation Birke. In June 1944, the Germans started to construct fortifications against a possible enemy advance from the south. The accidental death of "" Eduard Dietl on 23 June 1944 brought Lothar Rendulic to the command of the 20th Mountain Army.