Complex sentence: With the Anglo-Spanish war still raging English privateers were causing havoc and these were mainly London merchants who in a grinding war of attrition were still able to roam the coast of the Spanish New World.
Split into simpler sentences: With the Anglo-Spanish war still raging, English privateers were causing havoc. These were mainly London merchants who, in a grinding war of attrition, were still able to roam the coast of the Spanish New World.