Only twenty-years old when his father died in 1647, he nevertheless became 'Stadhouder'. In 1648 the Treaty of Muenster brought an end to the war with Spain, although Willem II continued behaving as though at war, an attitude making him close to his cousin,
Willem Frederik, 'Stadhouder' of Friesland.
The handsome Willem had been married even earlier, in 1641 when both were children, to the English Princess Mary Stuart. Ironically, Mary Stuart did not approve of her mother-in-law, Amalia, as she had been a lady-in-waiting to Mary's aunt, Elizabeth, 'the Winter Queen' of Bohemia. Temperamental, he enjoyed the good life with the English
nobles living in exile. Willem had been difficult with his teachers as well as with his
father, who had remarked: "He is a young boy who has done nothing and doesn't know what to do". However, he had a quick mind and, according to Johann de Witt, was "the ablest of his race". 
Willem was able and ambitious, while his cousin and brother-in-law, Willem Frederik, was more interested in war-fare. Also, Willem resented the Muenster peace-treaty for financial reasons; but once it was signed, he reduced his army and was more active in diplomacy. He wanted to retain the foreign officers of the discharged armies as this would keep the experienced officers for the smaller Dutch armies. However, the Dutch counties expected peace to be lasting and so objected as these officerss would be too costly. It appears that Willem II preferred to continue the struggle against Spain as he hoped to gain more of the Southern or Spanish Netherlands. After the execution of his father-in-law, King Charles I
of England, his wife's brothers came to Holland and Willem borrowed money from the city of Amsterdam to assist the future Charles II. The still-unresolved matter of the foreign officers now came to a head as the States of Holland, on their own authority, wanted to
dismiss them. Willem then arrested some of the deputies and imprisoned them at Loevestein. Cousin and brother-in-law Willem Frederik then appeared with an army, but the city of Amsterdam refused them entry. Civil war almost ensued when Amsterdam was surrounded by the armies of both Willem II and Willem Frederik. However, the city and the Prince then came to an agreement. In the future the city was to support the Prince in regards to his military requirements. Consequently, peace may have been restored but trust was destroyed.
Willem then tried to have the Dutch States accept Charles II's representative with Willem mediating between France and Spain. Consequently, the Spanish ambassador tried to establish Willem as Duke of Limburg, but these matters were to remain unresolved as Willem went hunting at Dieren and became ill. He arranged to be taken to The Hague where he died of smallpox, 6 November 1650, aged only twenty-four.
 

Source: Leo van de Pas