This was conclusively proved by the passing, at the instigation of Holland, of the _Acte de Survivance_ (April 19,1631).

This Act declared all the various offices held by the prince hereditary in the person of his five-year-old son.

He thus became, in all but name, a constitutional sovereign.
An expedition planned for the capture of Dunkirk at this time, spring 1631, proved too hazardous and was abandoned, but later in the year the Dutch sailors gave a signal proof of their superiority at sea.
Encouraged by the failure of the attempted attack on Dunkirk the government at Brussels determined on a counter-stroke.

A flotilla of 35 frigates, accompanied by a large number of smaller vessels to carry supplies and munitions and having on board a body of 6000 soldiers, set sail from Antwerp under the command of Count John of Nassau (a cousin of the stadholder) and in the presence of Isabel herself to effect the conquest of some of the Zeeland islands.

As soon as the news reached Frederick Henry, detachments of troops were at once despatched to various points; and about a dozen vessels were rapidly equipped and ordered to follow the enemy and if possible bring him to action.