Following flight training, he was posted to Jagdgeschwader 3 "Udet" (JG 3—3rd Fighter Wing) in 1942, operating on the Eastern Front.

Staffel (5th squadron) of Jagdgeschwader 3 "Udet" (JG 3—3rd Fighter Wing) with the rank of Unteroffizier (a junior non-commissioned officer) in May 1942.

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

Gruppe had just arrived at Pilsen, present-day Plzeň in the Czech Republic, after service in the Mediterranean theater.

Fliegerkorps (8th Air Corps), fighting on the left wing of Army Group South.

Gruppe of JG 3, was responsible for providing fighter escort to Junkers Ju 52 transport aircraft and Heinkel He 111 bombers shuttling supplies for the encircled German forces fighting in Stalingrad.

[8] Flying from Pitomnik Airfield, Grünberg claimed his last aerial victory of 1942 and eleventh in total on 27 December when he shot down an Ilyushin Il-2 ground-attack aircraft.

[9] Grünberg became an "ace-in-a-day" on 5 July 1943, the first day of Operation Citadel, the German offensive phase of the Battle of Kursk.

[11] His Bf 109 G-6 suffered engine failure following aerial combat with a Yakovlev Yak-1 fighter, forcing him to bail out.

[12] On 1 August 1943, Grünberg claimed his last aerial victory on the Eastern Front when he shot down an Ilyushin Il-2 ground-attack aircraft.

The Gruppe spent one-month training in northern Germany before they arrived at the Schiphol airfield near Amsterdam in the Netherlands on 12 September.

[16] On 20 December 1943, Hauptmann Heinrich Sannemann, at the time Grünberg's acting group commander, filed a special report requesting his promotion to wartime officer, based on his bravery before the enemy.

On 1 January 1944, Oberst Wolf-Dietrich Wilcke, wing commander of JG 3, concurred in the assessment and stated that Grünberg is well suited for promotion to Leutnant (second lieutenant).

On 25 November, the Gruppe was withdrawn from combat operations and relocated to Landsberg-Lech Airfield for conversion training to the Messerschmitt Me 262 jet aircraft.

Nowotny, a fighter pilot credited with 258 aerial victories and recipient of the Knight's Cross of the Iron Cross with Oak Leaves, Swords and Diamonds (Ritterkreuz des Eisernen Kreuzes mit Eichenlaub, Schwertern und Brillanten), had been assessing the Me 262 jet aircraft under operational conditions.

On 12 November 1944, the Oberkommando der Luftwaffe (OKL—Air Force High Command) ordered JG 7 "Nowotny" to be equipped with the Me 262.

[26][27] On 10 April, the USAAF Eighth Air Force sent 1,315 heavy bombers against German operations, attacking the airfields at Brandenburg-Briest, Rechlin-Lärz, Oranienburg, Neuruppin, Burg and Parchim.

[32][Note 2] Obermaier also states that he was credited with 82 aerial victories with 61 on the Eastern Front and 21 over the Western Allies.