In 1635 a formal and public alliance was declared between the United Provinces and France, and war broke out once more between Spain and the confederates. The operations which followed form part of the fourth phase of the Thirty Years' War, but we are only concerned here with their result with regard to the Netherlands. While the Dutch took Breda and concentrated near Maestricht, the French advanced through the Southern provinces towards Limburg, where they made their junction with their allies to proceed against Brussels. The Belgians had not answered the Franco-Batavian manifesto, inviting them to rebel, and gave whatever help they could to their Spanish governor, the Cardinal Infant Ferdinand. Students co-operated in the defence of Louvain, and the people showed the greatest loyalty during the campaign. They knew by now that they had very little good to expect from a Franco-Dutch protectorate and that even the shadow of independence they were allowed to preserve under the Spanish régime would be taken from them. Powerless to reconquer full independence, they preferred a weak rule which secured for them at least religious liberty to the strong rule of those whom they considered as foreigners and as enemies to their country.
Operations were pursued with alternating success until 1642, when Mazarin succeeded Richelieu as French Prime Minister. Mazarin favoured a more radical solution of the Netherlands difficulty. He persuaded Louis XIV that the possession of the left bank of the Rhine was essential to the safety of the kingdom, and aimed at the total annexation of the Belgian Provinces. The negotiations begun in that direction met with Dutch and English opposition and the curt refusal of Spain to renounce her rights on her Northern possessions. This new attitude of France brought about a rapprochement between Spain and the United Provinces, who began to fear Louis XIV's ambitious schemes. The two countries settled their difficulties by the treaty of Münster (1648), while, after a new series of defeats, culminating, in 1658, in the Battle of the Dunes, won by Turenne against Don Juan, Philip IV was finally obliged to submit to the treaty of the Pyrenees (1659).
The Dutch plenipotentiaries had practically a free hand in the settling of the Münster treaty. They acquired all the territories they claimed, and they only claimed the territories they wanted and which they already held. Their choice was dictated neither by territorial ambition nor by the desire to realize the unity of the Netherlands. They obtained, of course, the official recognition of their full independence and the maintenance of the closing of the Scheldt and of its dependencies. The annexation of Zeeland Flanders, henceforth known as Flanders of the States, ensured their position on the left bank of the stream, that of North Brabant with Bergen-op-Zoom, Breda and Bois-le-Duc, ensured the protection of their central provinces, while Maestricht, together with Fauquemont, Daelhem and Rolduc, secured their position on the Meuse. These were purely strategic annexations, prompted by strategic motives and by the desire to keep a firm hold on some key positions from which the United Provinces could check any attack, either from Spain or from France, with the least effort.