Complex sentence: The 18,000 French supported by 3,000 troops from the English Commonwealth besieged Dunkirk, Spain's greatest base for privateers and these privateers had wreaked havoc of English merchant shipping.
Split into simpler sentences: The 18,000 French supported by 3,000 troops from the English Commonwealth besieged Dunkirk. Dunkirk was Spain's greatest base for privateers and these privateers had wreaked havoc of English merchant shipping.