[5] When Seiler returned to Germany, he was credited with nine aerial victories and was one of the leading fighter pilots of the Condor Legion.

The Staffel was based at Herzogenaurach, equipped with the Messerschmitt Bf 109 D-1 and subordinated to the I. Gruppe (1st group) of Jagdgeschwader 51 (JG 51—51st Fighter Wing) under the command of Major Ernst Freiherr von Berg.

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

Seiler's unit was kept back and on 13 September, it formed the nucleus of the newly created I Gruppe of Jagdgeschwader 54 (JG 54—54th Fighter Wing) which had been placed under command of Major Hans-Jürgen von Cramon-Taubadel.

However he scored no further in the subsequent Battle of France, when his unit covered the Panzer advance through the Ardennes forests and later over the Dunkirk bridgehead.

On a bomber escort mission over Dover on 5 August 1940, Seiler claimed his third aerial victory, shooting down a Supermarine Spitfire over the English Channel.

In the fortnight prior, JG 54 had been moved to an airfield in Lindenthal near Rautenberg, East Prussia, present-day Uslowoje in Kaliningrad Oblast.

Tasked with supporting Army Group North in its advance through the Baltic states towards Leningrad, the unit began combat operations shortly afterwards.

By the end of September, he had 33 victories and his unit had finally settled down, establishing itself at Siverskaya, (about 60 kilometers (37 mi) south of Leningrad).

In spring 1942 Geschwaderkommodore (Wing Commander) Hannes Trautloft had the idea for fighter interceptions of Soviet night-harassment raids on moonlit nights.

In December though, Seiler took his III./JG 54 to Smolensk in the centre, and then soon after in early 1943 rotated back to the west as part of Adolf Galland's mis-guided plan to swap units between the western and eastern fronts in exchange for I./Jagdgeschwader 26 (JG 26—26th Fighter Wing).

Finally in March, they were transferred back to Oldenburg in northern Germany for further training and to stay on Defence of the Reich duties.

He replaced Hauptmann Gerhard Koall who temporarily led the Gruppe after Major Hans Philipp was transferred to take command of Jagdgeschwader 1 (JG 1—1st Fighter Wing) fighting in Defence of the Reich.

However immediately afterward he was himself shot and forced to bail out badly wounded over enemy territory east of Ponyri, midway between Orel and Kursk.

Also present at the award ceremony were Gerhard Barkhorn, for the Swords to his Knight's Cross, and Erich Hartmann, Walter Krupinski, Kurt Bühligen, Horst Ademeit, Hans-Joachim Jabs, Dr. Maximilian Otte, Bernhard Jope and Hansgeorg Bätcher from the bomber force, and the Flak officer Fritz Petersen, all destined to receive the Oak Leaves.

[20] Later in the year, on 8 August, he was appointed Geschwaderkommodore of the fighter-pilot training unit Jagdgeschwader 104 and served in this position until it was disbanded on 28 April 1945, just days before the end of World War II.

Released from Allied captivity in 1946, Seiler died on 6 October 1989 at the age of 80, in the town of Grafengehaig near Kulmbach, in Bavaria.