[6] He took his examen artium in 1912, graduated from business school in 1919 and entered law studies at the Royal Frederick University in 1921.

[8] According to a 1998 report on the Alta Battalion by the Norwegian Ministry of Health and Care Services, an obituary for Dahl, the source of which is not recorded, states that as a young lieutenant he participated with a division of the British Army in the Battle of the Somme in 1916.

[3][13] In 1941 Dahl became the first Norwegian to attend the Command and General Staff College at Fort Leavenworth, Kansas.

Once there, he assumed control of the front from the Soviets, commanding the Free Norwegian Forces that had followed him from Britain, locally raised militias, and recently-arrived police troops from Sweden.

Dahl on his part had concerns that the Soviet forces which had been stationed in Eastern Finnmark since the October 1944 Petsamo-Kirkenes Operation might not leave after the end of the war.

[3][4] In the period 1 September 1949 to 31 October 1950,[15] Dahl commanded the Independent Norwegian Brigade Group in Germany.