Complex sentence: In 1940, only Horn's Swedish nationality prevented their internment by the fascist Italian authorities, thus their first daughter, Mieke Marie, was born in Mariefred near Stockholm in June 1940.
Split into simpler sentences: In 1940, only Horn's Swedish nationality prevented their internment by the fascist Italian authorities. Thus their first daughter, Mieke Marie, was born in Mariefred near Stockholm in June 1940.