From 1937 to 1941, Hansteen served as Norwegian Military attaché to Helsinki. In this position he worked for stronger Norwegian support for Finland during the Winter War. After a short period in Stockholm, he was in 1942 surprisingly promoted directly from Major to Major General and appointed Commander in Chief of the Norwegian Armed Forces. He held this position until 1944 when then-Crown Prince Olav was made Commander in Chief. Hansteen served as Deputy Commander for the rest of World War II.