In 1585 the English intervened in the war, which to that point had spanned 17 years. Robert Dudley, earl of Leicester, came to the aid of the Netherlands with his army of 6,000 English troops. Although granted sweeping powers by the Netherlanders, Leicester was inept, and after the death of his lieutenant, Philip Sidney (1554–86), he withdrew. William’s son Maurice was named captain and admiral-general.

In 1589 Philip II ordered the duke of Parma to the aid of the French Catholics battling Henry of Navarre (1553–1610). Taking advantage of Parma’s absence, Maurice liberated Breda in a surprise attack. Maurice continued on to Zutphen and Deventer, where he won victories in June and July of 1591. He then faced Parma along the Waal River near Arnhem, but Philip again ordered Parma to France, this time to relieve Rouen, under siege by Henry of Navarre. Maurice now struck quickly by both water and road and captured Hulst and Nijmegen. His string of successes continued beyond the death of Parma in 1592 to the Battle of Turnhout on January 24, 1597. There the rebel forces, after making a 24-mile march in nine hours, routed the Spanish army under Count Jean de Rie of Varas. Over the next few years the Spanish made no headway in their attempts to regain control of the Netherlands.

In 1600 Maurice, under orders from the States General, invaded Flanders. His army crossed the Scheldt River and routed the Spanish blockaders of Ostend. On July 2 Maurice sent a detachment to Leffingham Bridge over the Yser River with orders to halt the approach of Albert of Austria, the new Spanish viceroy and son-in-law of Philip II, on his way to try his hand at conquering the rebels. The Spanish reached the bridge first and destroyed the Dutch detachment. Later that day the two armies engaged. Albert ordered his reserves into that fray, but by then it was too late for the weary Spanish. Exhausted by their long march of several days, the Spanish force collapsed under a general attack by the Dutch.

Between 1601 and 1604 the Spanish besieged Ostend. The Dutch, losing 20,000 men during the siege, fared better than the Spanish, who lost as many as 60,000 men due to casualties or disease. While the siege was under way both England and France negotiated peace terms with the Spanish, thereby leaving the Dutch alone in their struggle against the Spanish. The States General ordered Ostend to surrender on September 20, 1604.

Over the next three years the Dutch managed to retain control of the sea. After the victory of Admiral Jacob van Heemskerck at the Battle of Gibraltar, the Spanish agreed to a truce.