Complex sentence: Oglethorpe's letter left little doubt that the colony was part of an international conspiracy, one which not only planned to infiltrate and destroy the city of New York, but also to engage its Protestant citizens in religious warfare.
Split into simpler sentences: Oglethorpe's letter left little doubt that the colony was part of an international conspiracy. A conspiracy that not only planned to infiltrate and destroy the city of New York, but to engage its Protestant citizens in religious warfare.