From 1930 to 1936, Diesen was the second-in-command at the Navy Mining School. He was promoted to Commander in 1936. In 1937 he was appointed as chief of the Third Naval Defence District (Northern Norway), and in 1938 he became Commanding Admiral in Norway. He was also promoted to rear admiral. In 1940 Norway was invaded by Nazi Germany. Two months later, Diesen embarked the ship HNoMS "Fridtjof Nansen" with which he fled the country. The vessel left Norway on 10 June, arrived at Tórshavn, in the Faroe Islands, on 13 June and later sailed to the United Kingdom where she was made war ready. From now the Royal Norwegian Navy was used in the Allied warfare. He left his position as Commanding Admiral in August 1941, over disagreements with the government, and served in the United States until 1944. From 1945 to 1947 he was the chief of "Marinekommando Vestlandet".