Complex sentence: The archbishopric of Bremen, surrounding the city's territory, was occupied by Sweden since 1646/47, when Swedish forces had expelled the forces of Frederik II, Prince of Denmark, who was bishop of Bremen from 1637 to 1648.
Split into simpler sentences: The archbishopric of Bremen, surrounding the city's territory, was occupied by Sweden since 1646/47. Then, Swedish forces had expelled the forces of Frederik II, Prince of Denmark, who was bishop of Bremen from 1637 to 1648.