I was reading an article on Wikipedia and found this
Christie had long been a fan of detective novels, having enjoyed Wilkie Collins' The Woman in White and The Moonstone as well as Sir Arthur Conan Doyle's early Sherlock Holmes stories. She wrote her own detective novel The Mysterious Affair at Styles featuring Hercule Poirot, a former Belgian police officer noted for his twirly large "magnificent moustaches" and egg-shaped head. Poirot had taken refuge in Britain after Germany had invaded Belgium. Christie's inspiration for this stemmed from real Belgian refugees who were living in Torquay.
I want to know why the past perfect is used twice? Should it be "Poirot had taken refuge in Britain after Germany invaded Belgium."?