Complex sentence: When those governments abandoned the practice of issuing letters of marque to privateers against the Spanish treasure fleets and possessions in the later 16th century, many of the crews turned pirate and used the city as their main base during the 17th century.
Split into simpler sentences: When those governments abandoned the practice of issuing letters of marque to privateers against the Spanish treasure fleets and possessions in the later 16th century, many of the crews turned pirate. They continued to use the city as their main base during the 17th century.