[4] Following flight training,[Note 1] he was posted to I. Gruppe (1st group) of Zerstörergeschwader 52 (ZG 52—52nd Destroyer Wing) where he served with 3.

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

At the time ZG 52 was commanded by Hauptmann Wilhelm Lessmann and was based at Biblis where it was tasked with protecting Germany's western border in the Saar region during the "Phoney War".

[8] A week later, on 24 September 1939, his Messerschmitt Bf 109 109 D-1 was damaged in combat with a French Morane-Saulnier M.S.406, resulting in a forced landing at Bingen.

152 was ordered to Graz where it was reequipped with the Messerschmitt Bf 110 heavy fighter and was renamed, again becoming the I. Gruppe of ZG 52.

[6] During these campaigns, Grasser was promoted to Oberleutnant (first lieutenant) on 1 June 1940 and received the Iron Cross first Class (Eisernes Kreuz erster Klasse) on 7 July.

Prior to his posting to JG 51, Grasser had been destined to become a night fighter pilot which was not what he wanted to do.

Grasser also served as a test pilot, flying the Bf 109 T, the aircraft carrier variant, which were to be produced at Fieseler in Kassel.

[10] Grasser claimed his first aerial victory flying with JG 51 on 1 December 1940 when he shot down a Royal Air Force (RAF) Hawker Hurricane fighter near Ashford.

[11] In June 1941, JG 51 and the majority of the Luftwaffe were transferred to the Eastern Front in preparation for Operation Barbarossa, the invasion of the Soviet Union.

[13] On 11 October, the Gruppe was briefly detached from the Geschwaderstab of JG 51 and ordered east to Oryol.

Defending against this operation, the Soviet Air Forces (VVS—Voyenno-Vozdushnye Sily) attacked the Luftwaffe airfield at Bryansk on 5 July.

Gruppe of JG 51 was scrambled and claimed 46 Soviet aircraft shot down for the loss of Bf 109s damaged.

Gruppe had been withdrawn from the Eastern Front in early October 1942 and sent to Jesau in East Prussia, present day Yushny, Bagrationovsky District, for conversion to the Focke-Wulf Fw 190.

Conversion training began on 7 October and on 4 November, the unit received the order to convert back to the Bf 109 and to transfer to the Mediterranean theatre.

Gruppe of Jagdgeschwader 1 (JG 1—1st Fighter Wing) which was based at Quedlinburg and fighting in Defense of the Reich.