In 1640, as the Spaniards remained on the defensive, the stadholder entered Flanders and by a forced march attempted to seize Bruges.  His effort, however, was foiled, as was a later attempt to capture Hulst, when Frederick Henry and the States sustained a great loss in the death of the gallant Henry Casimir of Nassau, who was killed in a chance skirmish at the age of 29 years.  This regrettable event caused a vacancy in the stadholderates of Friesland and Groningen with Drente.  A number of zealous adherents of the House of Orange were now anxious that Frederick Henry should fill the vacant posts to the exclusion of his cousin, William Frederick, younger brother of Henry Casimir.  They urged upon the prince, who was himself unwilling to supplant his relative, that it was for the good of the State that there should be a unification of authority in his person; and at last he expressed himself ready to accept the offices, if elected.  The result of the somewhat mean intrigues that followed, in which Frederick Henry himself took no part, gave a curious illustration of the extreme jealousy of the provinces towards anything that they regarded as outside intrusion into their affairs.  The States-General ventured to recommend the Estates of Friesland to appoint the Prince of Orange; the recommendation was resented, and William Frederick became stadholder.  The Frieslanders on their part sent a deputation to Groningen in favour of William Frederick, and Groningen-Drente elected the Prince of Orange.  This dispute caused an estrangement for a time between the two branches of the House of Nassau, which was afterwards healed by the marriage of the Friesland stadholder with Albertine Agnes, a daughter of Frederick Henry.  From this union the present royal family of Holland trace their descent.

The military operations of the years 1641, 1642 and 1643 were dilatory and featureless.  Both sides were sick of the war and were content to remain on the defensive.  This was no doubt largely due to the fact that in rapid succession death removed from the stage many of those who had long played leading parts in the political history of the times.  Aerssens died shortly after his return from his successful mission to England in the autumn of 1641; and almost at the same time the Cardinal Infante Ferdinand, who during his tenure of the governor-generalship had shown great capacity and prudence both as a statesman and as a commander, expired.  In 1642, after eighteen years of almost autocratic rule, Richelieu passed away, his death (December 4, 1642) coming almost half-way between those of his enemy, the intriguing Marie de’ Medici (July 3,1642), and that of her son, Louis XIII (May 18, 1643).  Anne of Austria, the sister of the King of Spain, became regent in France; but this did not imply any change of policy with regard to the United Provinces, for Cardinal Mazarin, who, through his influence over the regent succeeded to the power of Richelieu, was a pupil in the school of that great