Complex sentence: While the main defendants were senior to London, he gained prominence worldwide by writing the book, which was was made into a film directed by Costa-Gavras, starring Yves Montand and Simone Signoret.
Split into simpler sentences: While the main defendants were senior to London, he gained prominence worldwide by writing the book. The book was made into a film directed by Costa-Gavras, starring Yves Montand and Simone Signoret.