All this time the Dutch had maintained the fiction that the States were not at war with France; but in January, 1745, the pressure of circumstances was too strong even for the weak-kneed Van der Heim and his fellow-statesmen, and a quadruple alliance was formed between England, Austria, Saxony and the United Provinces to maintain the Pragmatic Sanction.  This was followed in March by the declaration of war between France and the States.  Meanwhile the position of Austria had improved.  The Emperor Charles VII died on January 20; and his youthful successor Maximilian Joseph, in return for the restoration of his electorate, made peace with Maria Theresa and withdrew all Bavarian claims to the Austrian succession.  Affairs in Flanders however did not prosper.  The command-in-chief of the allied army had been given to the Duke of Cumberland, who was no match for such an opponent as Maurice de Saxe.  The Prince of Waldeck was in command of the Dutch contingent.

The provinces of Friesland, Groningen, Overyssel and Gelderland had repeatedly urged that this post should be bestowed upon the Prince of Orange; and the States-General had in 1742 offered to give William the rank of lieutenant-general in the army, but Holland and Zeeland steadily refused.  The campaign of 1745 was disastrous.  The battle of Fontenoy (May 11) resulted in a victory for Marshal Saxe over the allied forces, a victory snatched out of the fire through the pusillanimous withdrawal from the fight of the Dutch troops on the left wing.  The British infantry with magnificent valour on the right centre had pierced through the French lines, only to find themselves deserted and overwhelmed by superior forces.  This victory was vigorously followed up.  The Jacobite rising under Charles Edward, the young Pretender, had necessitated the recalling not only of the greater part of the English expeditionary force, but also, under the terms of the treaties between Great Britain and the United Provinces, of a body of 6000 Dutch.  Before the year 1745 had ended, Tournay, Ghent, Bruges, Oudenarde, Dendermonde, Ostend, Nieuport, Ath fell in succession into the hands of Marshal Saxe, and after a brave defence Brussels itself was forced to capitulate on February 19, 1746.

Van der Heim and the Republican conclave in whose hands was the direction of foreign affairs, dreading the approach of the French armies to the Dutch frontier, sent the Count de Larrey on a private mission to Paris in November, 1745, to endeavour to negotiate terms of peace.  He was unsuccessful; and in February, 1746 another fruitless effort was made, Wassenaer and Jacob Gilles being the envoys.  The French minister, D’Argenson, was not unwilling to discuss matters with them; and negotiations went on for some time in a more or less desultory way, but without in any way checking the alarming progress of hostilities.  An army 120,000 strong under Marshal Saxe found for some months no force strong enough to resist it.  Antwerp, Louvain, Mechlin, Mons, Charleroi, Huy and finally Namur (September 21) surrendered to the French.  At last (October 11) a powerful allied army under the command of Charles of Lorraine made a stand at Roucoux.  A hardly-fought battle, in which both sides lost heavily, ended in the victory of the French.  Liege was taken, and the French were now masters of Belgium.