He scored his first victory of the war on 10 January 1940, shooting down a French reconnaissance Potez southwest of Freiburg. He scored a second victory on 7 April west of Strasbourg. 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. Pulled out early, back to occupied Netherlands as the campaign wound down, I./JG 54 was then one of the first units to re-occupy the Pas de Calais, in early August 1940 in anticipation of the upcoming Battle of Britain. On a bomber escort mission over Dover on 5 August 1940, Seiler scored his third victory (a Spitfire), but was bounced by a Hurricane squadron, shot down and severely injured. Taking to his parachute over the English Channel, he was rescued and hospitalised, but was out of action for over 6 months.