In 1942 he took part in his last movie called "Der grosse König" (1942) as the young prince Heinrich before he was called up to the war and sent to the East front with the infantry regiment "Grossdeutschland".
His parents got divorced in 1928 and when the political situation in Germany came to a head, Douglas Sirk emigrated with his second wife Hilde Jary - a Jewess - to the USA. Klaus-Detlef Sierck stood with his mother in Germany.
The actor Klaus-Detlef Sierck was a popular child player in the 30s and 40s.