Complex sentence: The causes of the Dutch Revolt and the ensuing Eighty Years War, considered to have started in June 1568, were a number of incidents and frictions had accumulated between the Dutch provinces and their Habsburg overlord.
Split into simpler sentences: The causes of the Dutch Revolt include various events which led to the Eighty Years War (or the Dutch Revolt), considered to have started in June 1568. Preceding the war, a number of incidents and frictions had accumulated between the Dutch provinces and their Habsburg overlord.