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

[2] On 27 August 1940, Hahn was appointed Gruppenkommandeur (group commander) of I. Gruppe of Jagdgeschwader 3 (JG 3—3rd Fighter Wing).

[4] In preparation for Operation Barbarossa, the German invasion of the Soviet Union, the I. Gruppe moved to an airfield at Dub on 18 June 1941.

At the start of the campaign, JG 3 was subordinated to the V. Fliegerkorps (5th Air Corps), under command of General der Flieger Robert Ritter von Greim, which was part of Luftflotte 4 (4th Air Fleet), under command of Generaloberst Alexander Löhr.

These air elements supported Generalfeldmarschall Gerd von Rundstedt's Heeresgruppe Süd (Army Group South), with the objective of capturing Ukraine and its capital Kiev.