on 9 April 1940, he fought in the Norwegian Campaign at Valdres. At the time of the campaign he held the rank of captain. Aasnæs arrived in Valdres on 25 April 1940, and assumed command of Company 7, 2nd Battalion of the 10th Infantry Regiment. The day prior to Aasnæs' arrival, the company had been thrown back from their positions in heavy fighting with advancing German units, losing their commander. After leading the company in heavy fighting for several days, the battalion commander, Major Leonard Sæter, was wounded, and Aasnæs was promoted to command the entire 2nd Battalion on 30 April. As the Norwegian forces' situation became more desperate, Aasnæs and fellow battalion commander, Captain Olav B. Skaathun, agreed to merge the remains of their units in the Vestre Slidre area, in an attempt to continue resistance against the German advance. However, the commander of the Norwegian forces in Valdres, Colonel Gudbrand Østbye, realized his forces were in an unwinnable position, and ordered their capitulation on 1 May 1940. On 3 May 1940, the officers were sent by buses to Oslo, while the non-commissioned officers and men were sent by train to a prisoner-of-war camp at Hvalsmoen. The Norwegian prisoners of war from the Valdres front were released from captivity in groups during May and June 1940, the last officers being released in mid-June. In 1941 Aasnæs made his way to the United Kingdom and joined the exiled Norwegian forces there, initially assuming command of a company of the Norwegian Brigade in Scotland. In 1942 he was promoted to the rank of major. One of his former soldiers would later describe Aasnæs as "... a strict but good company commander." From February to August 1944 he served as chief of staff of the Norwegian Brigade in Scotland. During his time in exile, Aasnæs was involved in several conflicts with other exiled Norwegians. In addition to repeatedly criticising the Norwegian government in exile, he was one of a very few officers to criticise the exiled army's first commander, General Carl Gustav Fleischer, whom Aasnæs described as "... tired and worn by all the difficulties in the first time in England ...," and "... at the moment not fully able to build a Norwegian army in the United Kingdom ...." In the last months of the war, Aasnæs was angered by the Norwegian authorities decision to retain the Norwegian Brigade in Scotland,