Complex sentence: He began working as the assistant of "Dagens Nyheder's" foreign correspondent, the strongly anti-Nazi Nic Blædel (1882--1943), and whose work "Forbrydelse og Dumhed" (Crime and Stupidity) he edited and published in 1945.
Split into simpler sentences: He began working as the assistant of "Dagens Nyheder's" foreign correspondent, the strongly anti-Nazi Nic Blædel (1882--1943). Seidenfaden edited and published Blædel's work "Forbrydelse og Dumhed" (Crime and Stupidity) in 1945.