He was a member of the States of Holland and an opponent of William II, Prince of Orange, stadtholder of Holland and four other provinces. Together with the republican-minded brothers Cornelis and Andries de Graeff and their cousins Cornelis and Andries Bicker, De Witt supported the Peace of Münster. In May 1650 they proposed to reduce the size of the army, which was opposed by William as commander of the army.
