Flying with this wing, Mietusch claimed his first aerial victory on 31 May 1940 on the Western Front during the Battle of France.

[2] Jagdgeschwader 26 "Schlageter" (JG 26—26th Fighter Wing) was created on 1 May 1939 in Düsseldorf by renaming Jagdgeschwader 132 (JG 132—132nd Fighter Wing) and was commanded by Oberst Eduard Ritter von Schleich, a flying ace of World War I.

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

Mietusch, who was serving as a technical officer with the Geschwaderstab (headquarters unit), was replaced in this capacity by Leutnant Walter Horten on 14 December.

Staffel (7th squadron) of JG 26, he crashed his Messerschmitt Bf 109 E-3 (Werknummer 1202—factory number) during takeoff at Chièvres Airfield on 27 May.

Gruppe (3rd group), to which his squadron was subordinated, was commanded by Major Ernst Freiherr von Berg who was replaced by Hauptmann Adolf Galland on 11 June.

264 Squadron from the Royal Air Force (RAF), he shot down a Hawker Hurricane fighter near Dunkirk.

To counter this threat the Luftwaffe and the Regia Aeronautica (Italian Royal Air Force) were tasked with bombing raids in an effort to neutralise the RAF defences and the ports.

Staffel moved to Grottaglie airfield near Taranto in Apulia to support the Invasion of Yugoslavia by the Wehrmacht.

On the first day of the campaign, Mietusch claimed a Yugoslav Hawker Fury, which in fact was an Avia BH-33, in combat northeast of Podgorica.

1 Squadron of the South African Air Force (SAAF) covering the British retreat after Operation Battleaxe.

1 Squadron lost three Hurricanes, two were claimed by JG 27 and a third was shot down by Mietusch in combat southeast of Sidi Omar.

Defending against this "Circus", Mietusch shot down a Royal Australian Air Force (RAAF) No.

Staffel began conversion training to the then new Focke-Wulf Fw 190 fighter aircraft at their airfield in Ligescourt.

[25] Following combat with a Spitfire near Boulogne-sur-Mer on 13 October, Mietusch made an emergency landing in his Bf 109 F-4 (Werknummer 7220) at Saint-Omer.

[29] At the time, Mietusch dated Lieselotte Hays, a Luftwaffenhelferin (female air force helper).

The VIII Bomber Command flew a mission to Lille, targeting the locomotive works and steel factory.

Staffel was attached to I. Gruppe of JG 54 which was commanded by Hauptmann Hans Philipp and was based outside of Leningrad.

Following a stop at Heiligenbeil, the unit was based at the airbase at Krasnogvardeysk, present day Gatschina, which is located approximately 40 kilometers (25 miles) southwest of Leningrad.

The Staffel was scrambled at 05:25 and ordered to intercept a flight of Petlyakov Pe-2 bombers, escorted by Lavochkin-Gorbunov-Gudkov LaGG-3 fighters, on a mission to bomb a railway bridge at Narva.

Staffel claimed five aerial victories on this mission, two Pe-2 bombers and three LaGG-3 fighters shot down, including a Pe-2 and two LaGG-3s by Mietusch.

[42] In July, the Gruppe received the Bf 109 G-4 and G-6 variants and the pilots began familiarizing themselves with this aircraft type while flying in Defense of the Reich.

Defending against these attacks, Mietusch claimed his first aerial victory as Gruppenkommandeur when he shot down a B-17 10 km (6.2 mi) north of Hamburg.

Gruppe intercepted the bombers prior to their bomb run and Mietusch shot a B-17 from its combat box west of Oldenburg.

That day, the Eighth Air Force, formerly known as VIII Bomber Command, targeted the ball bearings works at Erkner near Berlin.

[56] On 12 May, the USAAF flew a mission against the German fuel industry, 886 bombers, escorted by 735 fighters, attacked six oil refineries.

Gruppe of JG 26 was based at Villacoublay Airfield and flew missions to Le Havre that day.

Shortly after, he ignored the warning of his wingman Unteroffizier Heinz Gehrke and was shot down in his Bf 109 G-6/U4 (Werknummer 440640) near Argentan-Alençon by another Spitfire from the Royal Canadian Air Force (RCAF) No.

[61] Near the German border to the Netherlands, Mietusch flight intercepted a squadron of North American P-51 Mustang fighters.

According to Caldwell's account, Mietusch never interacted with non-commissioned officers, overcoming his lack of self-confidence by an act of will.