On 1 March 1937, von Rettberg was appointed adjutant to OberstKurt-Bertram von Döring, the Geschwaderkommodore (wing commander) of Jagdgeschwader 134 "Horst Wessel" (JG 134—134th Fighter Wing).[2] On 1 April 1938, he was appointed Staffelkapitän of 1. Staffel (1st squadron), a squadron of I. Gruppe (1st group) of JG 134, which was based at Dortmund and commanded by OberstleutnantHermann Frommherz who was succeeded by HauptmannKarl Kaschka on 1 February 1939.[3] On 1 November 1938, I. Gruppe was renamed to I. Gruppe of Zerstörergeschwader 142 (ZG 142—142nd Destroyer Wing), a unit which became the I. Gruppe of Zerstörergeschwader 26 (ZG 26—26th Destroyer Wing) on 1 May 1939. In consequence, von Rettberg commanded the 1. Staffel of ZG 142, and later the 1. Staffel of ZG 26. The Gruppe was equipped with the Messerschmitt Bf 109 D-1 fighter.[4] During the prelude of World War II in August 1939, I. Gruppe of ZG 26 deployed to an airfield at Varel.[5]
Mathews and Foreman, authors of Luftwaffe Aces — Biographies and Victory Claims, researched the German Federal Archives and state that Rettberg is credited with approximately eight aerial victories. This number includes three claims on the Eastern Front and five on the Western Front.[9]
Braatz, Kurt[in German] (2005). Gott oder ein Flugzeug – Leben und Sterben des Jagdfliegers Günther Lützow [God or an Airplane – Life and Death of Fighter Pilot Günther Lützow] (in German). Moosburg, Germany: NeunundzwanzigSechs Verlag. ISBN978-3-9807935-6-8.
Fellgiebel, Walther-Peer[in German] (2000) [1986]. Die Träger des Ritterkreuzes des Eisernen Kreuzes 1939–1945 — Die Inhaber der höchsten Auszeichnung des Zweiten Weltkrieges aller Wehrmachtteile [The Bearers of the Knight's Cross of the Iron Cross 1939–1945 — The Owners of the Highest Award of the Second World War of all Wehrmacht Branches] (in German). Friedberg, Germany: Podzun-Pallas. ISBN978-3-7909-0284-6.
Mathews, Andrew Johannes; Foreman, John (2015). Luftwaffe Aces — Biographies and Victory Claims — Volume 3 M–R. Walton-on-Thames, UK: Red Kite. ISBN978-1-906592-20-2.
Obermaier, Ernst (1989). Die Ritterkreuzträger der Luftwaffe Jagdflieger 1939 – 1945 [The Knight's Cross Bearers of the Luftwaffe Fighter Force 1939 – 1945] (in German). Mainz, Germany: Verlag Dieter Hoffmann. ISBN978-3-87341-065-7.
Patzwall, Klaus D.; Scherzer, Veit (2001). Das Deutsche Kreuz 1941–1945 Geschichte und Inhaber Band II [The German Cross 1941–1945 History and Recipients Volume 2] (in German). Norderstedt, Germany: Verlag Klaus D. Patzwall. ISBN978-3-931533-45-8.
Prien, Jochen; Stemmer, Gerhard; Rodeike, Peter; Bock, Winfried (2000). Die Jagdfliegerverbände der Deutschen Luftwaffe 1934 bis 1945—Teil 1—Vorkriegszeit und Einsatz über Polen—1934 bis 1939 [The Fighter Units of the German Air Force 1934 to 1945—Part 1—Pre-War Period and Action over Poland—1934 to 1939] (in German). Eutin, Germany: Struve-Druck. ISBN978-3-923457-54-0.
Prien, Jochen; Stemmer, Gerhard; Rodeike, Peter; Bock, Winfried (2001). Die Jagdfliegerverbände der Deutschen Luftwaffe 1934 bis 1945—Teil 2—Der "Sitzkrieg"—1.9.1939 bis 9.5.1941 [The Fighter Units of the German Air Force 1934 to 1945—Part 2—The "Phoney War"—1 September 1939 to 9 May 1940] (in German). Eutin, Germany: Struve-Druck. ISBN978-3-923457-59-5.
Scherzer, Veit (2007). Die Ritterkreuzträger 1939–1945 Die Inhaber des Ritterkreuzes des Eisernen Kreuzes 1939 von Heer, Luftwaffe, Kriegsmarine, Waffen-SS, Volkssturm sowie mit Deutschland verbündeter Streitkräfte nach den Unterlagen des Bundesarchives [The Knight's Cross Bearers 1939–1945 The Holders of the Knight's Cross of the Iron Cross 1939 by Army, Air Force, Navy, Waffen-SS, Volkssturm and Allied Forces with Germany According to the Documents of the Federal Archives] (in German). Jena, Germany: Scherzers Militaer-Verlag. ISBN978-3-938845-17-2.