"Gruppe" (2nd group) of "Jagdgeschwader" 26 "Schlageter" (JG 26—26th Fighter Wing), named after the Nazi martyr Albert Leo Schlageter, where he was posted to the 6. "Staffel" where he made his first flight. From 1 September to 15 October 1939, Schröer attended an air observer course at the "Aufklärungsfliegerschule" (Reconnaissance Flying School) at Hildesheim. Schröer then continued his pilot training at the flight schools in Kamp and Schafstädt . On 16 May 1940, he completed his flight training with "Flugkommando" 23 (Flight Commando) in Braunschweig. During this training period, he was promoted to "Feldwebel" (sergeant) on 1 December 1939. Schröer then received fighter pilot training at the "Jagdfliegerschule" 1 (fighter pilot school) at Werneuchen. There he learned to fly the Arado Ar 68 and Ar 96, the Messerschmitt Bf 108 and Bf 109, the Bücker Bü 131, the Focke-Wulf Fw 56, and the Heinkel He 45 and He 51. From 22 July to 17 August 1940, Schröer completed his fighter pilot training with the 2. "Staffel" of "Ergänzungsjagdgruppe" Merseburg, a supplementary training unit based in Merseburg. World War II in Europe began on Friday 1 September 1939 when German forces invaded Poland. On 20 August 1940, Schröer was transferred to the 2. "Staffel" of "Jagdgeschwader" 27 (JG 27—27th Fighter Wing), a front line fighter unit. JG 27 at the time was under the command of "Oberst" (Colonel) Max Ibel and based in Plumetot, France on the Channel Front. There, JG 27 was subordinated to "Jagdfliegerführer" 3 (Jafü 3), the fighter force commander of "Luftflotte" 3 under the command of "Generalfeldmarschall" (Field Marshal) Hugo Sperrle, and fought in the Battle of Britain (10 July – 31 October 1940). On 28 August 1940, JG 27 relocated from the vicinity of the Cotentin Peninsula to an airfield at Peuplingues, about southwest of Calais and subsequently was placed under the command of "Jagdfliegerführer" 2.