Aliens had to register for the World War I draft as evidenced by this card for Garrit A. Fecht from Varna, Illinois.
Anton's family was investigated during WW1 for "un-American" activity. His draft registration was not mentioned in the investigation. This investigation was the focus of issue 29 of Casefile Clues.
Subscribe in the next two days and I'll send you issue 29 at no extra charge and you can see what happened to Anton and his family.