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

That day, I. Gruppe moved to Bonn-Hangelar Airfield [de], protecting the German border during the "Phoney War".

[7] In preparation for Operation Sea Lion (Unternehmen Seelöwe), the proposed amphibious invasion of Great Britain, I. Gruppe of JG 52 moved to an airfield at Coquelles near the English Channel on 3 August 1940.

Here on 24 August, Leesmann claimed his first aerial victory, a Royal Air Force (RAF) Supermarine Spitfire shot down east of Margate.

[9] On 15 September, also known as Battle of Britain Day, Leesmann claimed two Hawker Hurricane fighters over the Thames Estuary on fighter escort mission for Junkers Ju 88 bombers heading for London, taking his total to ten aerial victories.

Here on 30 June, Leesmann claimed two Bristol Blenheim bombers shot down northwest of Texel, taking his total to 22.

In support for Operation Typhoon, also known as the Battle of Moscow, the Gruppe moved to an airfield named Ponyatovka, located approximately 30 kilometers (19 miles) southwest of Roslavl, on 2 October.

[20] On 6 November, Leesmann was severely wounded in combat when his Messerschmitt Bf 109 F-2 (Werknummer 9181—factory number) was hit by enemy fire resulting in an emergency landing at Ruza.

At the time, I. Gruppe was based at Olmütz, present-day Olomouc in the Czech Republic, where they received new Bf 109 F-4 aircraft.

[24][25][26] On 25 July during Blitz Week, a period of United States Army Air Forces (USAAF) aerial bombardment during the 1943 Combined Bomber Offensive, the USAAF VIII Bomber Command targeted Hamburg and Warnemünde.

Gruppe was scrambled at 16:15 and vectored to a point of intercept over the Elbe estuary where they encountered a formation of approximately 100 Boeing B-17 Flying Fortress bombers.

Leesmann was killed in action when his Bf 109 crashed into the North Sea in an area southeast of Heligoland.