In 1940, following the German invasion of Norway General Fleischer was appointed commander-in-chief of the Norwegian armed forces in North Norway. At the time of the attack on 9 April 1940 Fleischer was at Vadsø in Finnmark as part of an inspection journey together with his chief-of-staff, Major Odd Lindbäck-Larsen. When message of the invasion reached him the area was in the middle of a ferocious blizzard. Due to the extreme weather Fleischer could not leave Vadsø either by Hurtigruten ship or naval aircraft, and had to stay overnight. County Governor of Finnmark Hans Gabrielsen invited Fleischer to stay at the governor's mansion. After discussing the situation with Gabrielsen, Fleischer managed to set off for Tromsø the next day, arriving there by M.F.11 naval aircraft after flying in terrible conditions. From Tromsø he issued orders for a total civilian and military mobilization and declared Northern Norway a theatre of war. He handed over most of the civilian powers to the respective County Governors in Troms and Finnmark, Hans Gabrielsen taking all civilian power in Northern Norway after the death of the County Governor of Troms a few days after the invasion. Fleischer's strategic plan was to first wipe out the German forces at Narvik and then transfer his division to Nordland to meet a German advance from Trøndelag. Fleischer valued offensive actions against enemy forces, using the unique nature of the Norwegian terrain to carry out attacks against an enemy's flanks and rear. General Fleischer had already in 1934 opposed the concept of fighting delaying actions while waiting for Allied reinforcements, a tactic on which General Otto Ruge relied during his defence of the vital Eastern Norway region.