He claimed 17 aerial victories on the Eastern Front and 47 over the Western Allies, including nine four-engined heavy bombers.

He claimed his first aerial victory on 22 June 1941, the first day of Operation Barbarossa, the German invasion of the Soviet Union.

Harder was born on 13 June 1918 in Swinemünde in the Province of Pomerania within the German Empire, present-day Świnoujście, Poland.

[1] On 1 October 1939, Harder joined the military service of the Luftwaffe as a Fahnenjunker (cadet) and was trained as a fighter pilot.

[Note 1] On 1 February 1941, Harder was promoted to Leutnant (second lieutenant) and in March was posted to the Gruppenstab (headquarters unit) of III.

[3][Note 2] While flying with the Gruppenstab, Harder made a crash landing in his Messerschmitt Bf 109 F-2 (Werknummer 8085—factory number) at the Maldegem Airfield.

Two days later, his aircraft sustained damage in aerial combat with Royal Air Force (RAF) Supermarine Spitfire fighters over the English Channel.

On 21 June, the Geschwaderkommodore (wing commander) of JG 53 and its Gruppenkommandeure were summoned to nearby Suwałki, where Generalfeldmarschall (field marshal) Albert Kesselring gave the final instructions for the upcoming attack.

[8] Thad day, Harder claimed his first aerial victory, shooting down an I-17 fighter, an early war designation for the Mikoyan-Gurevich MiG-1.

[9] On 25 September, Harder got disoriented and his Bf 109 F-2 (Werknummer 9212) ran out of fuel resulting in an emergency landing near Novgorod.

[13] On 13 October, Harder claimed a Curtiss P-40 Warhawk fighter on a morning escort mission for Junkers Ju 87 dive bombers.

Staffel flew from Sciacca on escort mission for three military transports heading for Tunesia without engaging in combat.

Gruppe moved to a makeshift airfield at Arlena di Castro located southeast of Lake Bolsena.

[21] Escorting German ground attack aircraft on 14 February, Harder shot down a Lockheed P-38 Lightning.

[22] The following day, Harder was informed that he had been appointed Gruppenkommandeur of I. Gruppe of JG 53, replacing Major Friedrich-Karl Müller who was transferred.

Prior to taking his new command, Harder met with Oberst Günther Freiherr von Maltzahn, the Jagdfliegerführer Oberitalien (Fighter Leader Northern Italy), in Udine where he received more specific orders.

[26] On 25 April, Harder attacked a four-engined bomber formation and shot one of them down and rammed another after his aircraft was damaged thereby claiming his aerial victories number 49 and 50.

[31] On 21 January, Harder was again ordered to Berlin to take command Jagdgeschwader 11 (JG 11—11th Fighter Wing).

[30] In early February, JG 11 predominately flew ground support missions in the area of Kienitz, present-day part of Letschin, where Soviet forces had created the first bridgehead across the Oder.

[35] On 17 February, Harder and his wingman Leutnant Hans Jung flew an aerial reconnaissance mission over the Oder.

The piston of cylinder 12 had penetrated the engine block, escaping toxic fumes thus intoxicated Harder who then lost control of the aircraft.

[38]Obermaier also lists him with 64 aerial victories, 17 over the Eastern Front and 47 in the Mediterranean theater and Western Allies, including nine four-engined heavy bombers.