On 1 April 1938, he was transferred to the Flughafenbetriebskompanie (Airport Operation Company) of Jagdgeschwader 132 (JG 132—132nd Fighter Wing) to Düsseldorf, serving with the ground personnel.

[5] JG 27 at the time was under the command of Oberst (Colonel) Max Ibel and based in Plumetot, France on the Channel Front.

[5] On 28 August 1940, JG 27 relocated from the vicinity of the Cotentin Peninsula to an airfield at Peuplingues, about 6 kilometers (3.7 mi) southwest of Calais and subsequently was placed under the command of Jagdfliegerführer 2.

[6] Operating over the English Channel and southern England, Schröer flew his first combat missions and claimed three aerial victories which were not confirmed.

Staffel were temporarily sent to Sicily where they flew missions against Malta, protecting the German naval convoys taking the Afrika Korps to Tripoli.

[15] Schröer was credited with his first aerial victory on 19 April, a Royal Air Force (RAF) Hawker Hurricane fighter of the No.

[17] In this encounter, Schröer was shot down in his Bf 109 E-7 (Werknummer 3790—factory number), resulting in a forced landing at Ayn al-Ġazāla.

[20] Two days later, on 21 April, I. Gruppe engaged in combat with Hurricane fighters over Tobruk, claiming one victory by Oberfeldwebel Albert Espenlaub for the loss of Unteroffizier Hans Sippel who was killed in action.

[12] In support of German ground forces on 14 September, Schröer claimed his seventh aerial victory when he shot down a Hurricane.

[30] On 1 March, when Schröer became adjutant in I. Gruppe of JG 27 learning command under the experienced Eduard Neumann, they were back at Martuba, east of Derna.

[12] Schröer claimed his first aerial victory while flying with the Gruppenstab (headquarters unit) on 30 May over a P-40 northeast of Bir Hacheim during the Battle of Gazala.

On 8 July, the Gruppe moved to a makeshift airfield named Quotaifiya, located approximately 50 kilometers (31 miles) east of Fuka.

Gruppe moved to an airfield named Haggag el Qasaba located approximately 35 kilometers (22 miles) southeast of Mersa Matruh.

Aerial combat increased again on Rommel launched Unternehmen Brandung (Operation Surf) leading to the Battle of Alam el Halfa.

Defending against this attack, Schröer claimed his 60th aerial victory when he shot down a Consolidated B-24 Liberator heavy bomber between Sollum and Benghazi.

Gruppe were withdrawn from North Africa and was ordered to regroup on Crete while the ground personnel were reassigned to Jagdgeschwader 77 (JG 77—77th Fighter Wing).

RAF records show that two Martin B-26 Marauder bombers were lost on 15 February and the time of the claims filed by Schröer.

Authors Prien, Rodeike and Stemmer argue that at the time the B-26 was a new and unknown aircraft type to Schröer while the date discrepancy cannot be explained.

[60] The Gruppe was up against complete Allied air superiority and had the hopeless task of trying to protect transport aircraft making desperate evacuation flights of remaining wounded and specialists out of the beleaguered Afrika Korps, now bottled up in Tunis.

Gruppe flew fighter protection for the cargo ship SS San Antonio (6,013 GRT), escorted by a torpedo boat, heading for Bizerte.

[65] In support of the German retreat from North Africa on 11 May, Schröer claimed a Boeing B-17 Flying Fortress heavy bomber near Cape Bon and an escorting P-38 shot near Marsala.

Gruppe of JG 27 was ordered to relocate to Lecce, located at the heel of the Italian Peninsula where they arrived on 20 June.

Gruppe flew its first operational combat mission when the USAAF VIII Bomber Command targeted Stuttgart.

[77] On 11 January 1944, VIII Bomber Command targeted German aircraft production in central Germany, attacking the cities Halberstadt, Magdeburg, Oschersleben and Braunschweig.

[82] On 24 May, Schröer claimed a North American P-51 Mustang fighter and two B-17s, making him the 73rd Luftwaffe pilot to achieve the century mark.

[83][84] When Allied forces launched Operation Overlord, the invasion of German-occupied Western Europe on 6 June, III.

[82] But the worsening situation and the intense pressure was taking its toll, and he was sent on a month's stress-leave in early June just as Allied attention turned to Normandy, possibly saving his life as the unit took very heavy losses in France.

[89] The Geschwader was deployed in eastern Germany, initially subordinated to Luftflotte 6 and then under Luftwaffenkommando Nordost, where it fought over the lower Oder in the Battle of the Oder–Neisse.

Schröer presented evidence of the conferment which was confirmed by the Gemeinschaft der Jagdflieger (Association of German Armed Forces Airmen).

[93] Spick and Obermaier also list Schröer with 114 enemy aircraft shot down claimed in 197 combat missions, the majority of which on the Western Front, including 61 in North Africa and 22 in Italy.