In the negotiations for a general peace between all the contending powers in the Thirty Years' War, which dragged on their slow length from 1643 to 1648, the stadholder became more and more convinced that it was in the interest of the Dutch to maintain Spain as a counterpoise to the growing power of France, and to secure the favourable terms, which, in her extremity, Spain would be ready to offer.
At first, however, there was no breach in the close relations with France; and Frederick Henry, though hampered by ill-health, showed in his last campaigns all his old skill in siege-craft.

By the successive captures of Hertogenbosch, Maestricht and Breda he had secured the frontiers of the republic in the south and south-east.

He now turned to the north-west corner of Flanders.

In 1644 he took the strongly fortified post of Sas-van-Gent, situated on the Ley, the canalised river connecting Ghent with the Scheldt.

In 1645 he laid siege to and captured the town of Hulst, and thus gained complete possession of the strip of territory south of the Scheldt, known as the Land of Waes, which had been protected by these two strongholds, and which has since been called Dutch Flanders.
Very shortly after the capitulation of Hulst, the ambassadors plenipotentiary of the United Provinces set out (November, 1645) to take their places at the Congress of Muenster on equal terms with the representatives of the Emperor and of the Kings of France and Spain.
The position acquired by the Dutch republic among the powers of Europe was thus officially recognised _de facto_ even before its independence had been _de jure_ ratified by treaty.