On graduation he was appointed second lieutenant at the Vestingartillerie (garrison artillery) in the Royal Netherlands Army, but returned to the KMA as a lecturer in 1900 at the age of 28.

[2] In the build-up to World War II, he began to give lectures for the conservative Alliance for National Reconstruction (Verbond voor Nationaal Herstel) led by Van Gybland Oosterhoff.

[2] In 1937, Seyffardt became a member of the National Socialist Movement in the Netherlands, and started writing articles for their publication Volk en Vaderland.

[2] On 28 June 1941, Arnold Meijer of the fascist Black Front made a proposal in Nederlandsch Dagblad, pleading for a joint establishment of a separate Dutch Legion that would take part in the fight against "the Russian Bolshevism”.

After training in Hamburg and East Prussia, in November 1941 it was ordered to the Eastern Front near Leningrad, under the overall command of Army Group North.

[2] After approval from the Dutch government in exile, on the evening of Friday 5 February 1943, after answering a knock at his front door in The Hague, Seyffardt was shot twice by student Jan Verleun who had accompanied Dr. Kastein on the mission.

On 7 February, CS-6 shot fellow institute member “Gemachtigde voor de Volksvoorlichting” (Attorney for the national relations) Hermannus Reydon and his wife.

[2] The gun used in this attack had been given to Dr. Kastein by Sicherheitsdienst (SD) agent Van der Waals, and after tracking him back through information,[clarification needed] arrested him on 19 February.