William was Duke of Guelders, as William I, from 1377 and Duke of Jülich, as William III, from 1393. William was known for his military activities, participating in the Prussian crusade five times and battling with neighbors in France and Brabant throughout his rule. His allies included Holy Roman Emperors, Charles IV and Wenceslaus, Richard II of England, and Conrad Zöllner von Rothenstein, the Grand Master of the Teutonic Knights. During his reign the duchies of Guelders and Jülich were temporarily unified.

The Duchy of Jülich comprised a state within the Holy Roman Empire from the 11th to the 18th centuries. The duchy lay west of the Rhine river and was bordered by the Electorate of Cologne to the east and the Duchy of Limburg to the west. It had territories on both sides of the river Rur, around its capital Jülich – the former Roman Iuliacum – in the lower Rhineland. The duchy amalgamated with the County of Berg beyond the Rhine in 1423, and from then on also became known as Jülich-Berg. Later it became part of the United Duchies of Jülich-Cleves-Berg.