After the death of Frederick Henry in March 1647, his son, the Stadtholder William II of Orange, attempted to extend the power of the stadtholderate particularly through maintaining the size of the army, which he commanded and using his supporters in six provinces to outvote Holland, the most prosperous province, in the States General. Following the end of the Eighty Years' War and the execution of his father-in-law, Charles I, William attempted to support the English Royalist cause to an extent that gave concern to even his own followers, and which involved him in disputes with the more committed republicans, particularly those in Holland. The execution of Charles outraged the Orangists, and the Dutch republicans that had attempted to save Charles's life, but the execution did not prevent the States General from continuing a policy of broad neutrality, dealing unofficially with the English parliament while allowing Royalist envoys into the country. The Commonwealth and the Dutch Republic had many things in common: they were both republican and Protestant and many members of States General sympathised with the aims of the English parliamentarians and, while strongly against its regicide, supported a pragmatic policy of neutrality, in opposition to the Royalist-supporting Stadtholder. The impasse between the two sides ended with the death of William II in November 1650. However his attempts to involve the Netherlands in action against the English Commonwealth in support of the exiled Charles II, which could have led at least to limited hostilities and possibly outright war, and it did lead to a republican reaction. Shortly before his death, William attempted to gain control of Amsterdam by imprisoning six leading members of the States of Holland from the city, but they were released when he died. These six led the province of Holland to assume the leadership of the republican movement, which would recognize the Netherlands as a free republic without a Stadholder. The resulting First Stadtholderless period began when William II died in 1650, although it was not until January 1651 that the last of the seven provinces agreed to it.