The Duke of Cleves was a title held by the rulers of the Duchy of Cleves, located in present-day Germany. The duchy emerged in the 14th century and became significant during the late Middle Ages. It was known for its strategic location and played a role in the politics of the Holy Roman Empire. One notable Duke of Cleves was John III, who ruled in the 16th century. His marriage to Mary I of England was politically significant, as it aimed to strengthen ties between England and the Protestant states of the Holy Roman Empire. The duchy eventually became part of the larger territory of Prussia.