World War II in Europe began on Friday 1 September 1939 when German forces invaded Poland. At the time, III. "Gruppe" was under the command of "Hauptmann" (Captain) Werner Mölders and had just been formed at Wiesbaden-Erbenheim on 26 September 1939. Müller's commanding officer with 8. "Staffel" was "Oberleutnant" (First Lieutenant) Hans von Hahn. Müller flew combat missions during the Phoney War period and received the Iron Cross 2nd Class () on 17 April 1940. He claimed his first aerial victory during the Battle of France. On 27 May 1940, during the Dunkirk evacuation, he shot down an Armée de l'Air Curtiss Hawk in the vicinity of Amiens. On 10 June, he was credited with the destruction of three French Morane-Saulnier M.S.406 fighter aircraft. The "Geschwader" flew its last combat missions in the French Campaign on 19 June. On 20 June, III. "Gruppe" provided fighter cover in the vicinity of Compiègne during the armistice negotiations which was signed on 22 June.