He joined the military service of the Wehrmacht, at first in the infantry, before transferring to the Luftwaffe where he was trained as a fighter pilot.

Flying with this wing, Dortenmann claimed his first aerial victory on 6 February 1944 over a Soviet Air Forces fighter aircraft.

In mid-1944, elements of JG 54 were moved to the Western Front, fighting in the aerial battles of the Normandy invasion.

This unit was renamed and reorganized a few times and became part of Jagdgeschwader 26 "Schlageter" (JG 26—26th Fighter Wing) In February 1945.

He graduated with his Abitur (diploma) in 1939 and volunteered for military service as an officer candidate of the reserves with the Heer (German Army).

[4] World War II in Europe began on Friday 1 September 1939 when German forces invaded Poland.

The division fought in the Battle of France and breached the Maginot Line near Wörth am Rhein between 19 and 24 June 1940.

During the Battle of France, Dortemann was awarded the Iron Cross 2nd Class (Eisernes Kreuz zweiter Klasse) on 2 June.

[6] On 1 June 1941, Dortenmann was posted to the Flugzeugführerschule A/B 2 (FFS A/B 2—flight school for the pilot license) in Demblin, present-day Dęblin.

On 9 June 1942, he was transferred to the Flugzeugführerschule A/B 116 (FFS A/B 116—flight school) in Göppingen, completing his Flugzeugführerabzeichen (Pilot's Badge) on 15 January 1943.

The unit was initially based at Bad Aibling but then relocated to Chateauroux-Deols Airfield in central France.

[10] On 21 July, Dortenmann was posted to Ergänzungs-Jagdgruppe Ost, a supplementary training unit for fighter pilots destined to serve on the Eastern Front which was then based at Toulouse Airfield.

[12] On 9 August, Dortenmann was instructed to conduct a training flight in a Focke-Wulf Fw 190, climbing to an altitude of 10,000 meters (33,000 feet).

[16] The Gruppe was based at Orsha and Vilnius, on the northern sector of the Eastern Front, flying the Fw 190 A-4, A-5 and A-6.

[18] On 14 January 1944, Soviet forces launched the Leningrad–Novgorod Offensive, an attack on the German Army Group North with the objective to lift the Siege of Leningrad.

[20] On 28 February, his Fw 190 A-6 (Werknummer 550885—factory number) was hit in the engine during aerial combat with La-5 fighters which resulted in a crash landing.

[22] Dortenmann was presented with the Iron Cross 1st Class (Eisernes Kreuz erster Klasse) by the Gruppenkommandeur, Hauptmann Horst Ademeit, and was sent on home leave to marry his bride-to-be Ingeborg.

Gruppe of JG 54, under the command of Hauptmann Reinhard Seiler, was the first unit to relocate to the Western Front in February 1943.

Gruppe was withdrawn from Normandy and returned to Germany for conversion training to the new inline engine powered Fw 190 D-9, the first unit to receive this aircraft.

[28][29][30] Dortenmann claimed his only four-engined bomber destroyed on 2 November when he shot down a Boeing B-17 Flying Fortress in combat near Osnabrück.

Arriving at Varrelbusch Airfield, the pilots were welcomed by JG 26 commanding officer Oberstleutnant Josef Priller.

Dortenmann's Staffel was scheduled to form part of a third wave ordered to conduct low-level fighter patrols over the front.

[37][38] Flying at an altitude of approximately 50–150 metres (160–490 feet), the attack force sustained losses during the approach to the target area, largely due to anti-artillery fire.

The airfield at Gimbergen was almost completely abandoned, the damage inflicted was minimal, and the losses sustained were significant.

Gruppe of JG 54 alone suffered five pilots killed or missing in action, plus four further taken prisoner of war, making the mission a total failure.

The objective was to meet up with a dozen Panzerblitz equipped aircraft, providing fighter escort on an anti-tank mission.

The secondary objective was to determine whether the East-West Axis—a street in central Berlin built as a triumphal avenue which in parts is now the Straße des 17.

[49] During the meeting, Hitler promoted von Greim to Generalfeldmarschall (field marshal) and appointed him Commander-in-Chief of the Luftwaffe.

[53] On 4 May, Dortenmann led a flight of nine Fw 190s from I. Gruppe on a combat air patrol over the Kiel Canal.

[54] Dortenmann had claimed 18 aerial victories flying the Fw 190 D-9, making him the most successful Luftwaffe fighter pilot to have flown this aircraft in combat.