One result of the fall of Maestricht was a renewal on the part of the Archduchess Isabel of negotiations for peace or a long truce. On the authority of Frederick Henry's memoirs the terms first offered to him in camp were favourable and might have been accepted. When, however, the discussion was shifted to the Hague, the attitude of the Belgic representatives had stiffened. The cause was not far to seek, for on November 6, 1632 the ever-victorious Gustavus Adolphus had fallen in the hour of triumph in the fatal battle of Luetzen. The death of the Swedish hero was a great blow to the Protestant cause and gave fresh heart to the despondent Catholic alliance. The negotiations dragged however their slow length along, the chief point of controversy being the old dispute about freedom to trade in the Indies. On this point agreement was impossible. Spain would yield nothing of her pretensions; and the Hollanders would hear of no concessions that threatened the prosperity of the East and West India Companies in which so many merchants and investors were deeply interested. Any admission of a Spanish monopoly or right of exclusion would have spelt ruin to thousands. The diplomatic discussions, however, went on for many months in a desultory and somewhat futile manner; and meanwhile though hostilities did not actually cease, the campaign of 1633 was conducted in a half-hearted fashion. The death of Isabel on November 29, 1633, shattered finally any hopes that the peace party in the Provinces (for there was a strong peace party) might have had of arriving at any satisfactory agreement. By the decease of the arch-duchess, who had been a wise and beneficent ruler and had commanded the respect and regard not only of her own subjects but of many northerners also, the Belgic provinces reverted to the crown of Spain and passed under the direct rule of Philip IV. The Cardinal Infante Ferdinand, fresh from his crushing victory over the Swedes at Noerdlingen, came as governor to Brussels in 1634, at the head of considerable Spanish forces, and an active renewal of the war in 1635 was clearly imminent.
In these circumstances Frederick Henry determined to enter into negotiations with France for the conclusion of an offensive and defensive alliance against Spain, the common enemy. He had many difficulties to encounter. The Estates of Holland, though opposed to the terms actually offered by the Brussels government, were also averse to taking any step which shut the door upon hopes of peace. Richelieu on his side, though ready, as before, to grant subsidies and to permit the enrolment of French regiments for the Dutch service, shrank from committing France to an open espousal of the Protestant side against the Catholic powers. The stadholder, however, was not deterred by the obstacles in his way; and the diplomatic skill and adroitness of Aerssens, aided by his own tact and firmness of will, overcame the scruples of Richelieu. The opposition of the Estates of Holland, without whose consent no treaty could be ratified, was likewise surmounted. Adrian Pauw, their leader, was despatched on a special embassy to Paris, and in his absence his influence was undermined, and Jacob Cats was appointed Council-Pensionary in his stead. In the spring of 1635 a firm alliance was concluded between France and the United Provinces, by which it was agreed that neither power should make peace without the consent of the other, each meanwhile maintaining a field force of 25,000 foot and 5000 horse and dividing conquests in the Southern Netherlands between them. This treaty was made with the concurrence and strong approval of the Swedish Chancellor, Oxenstierna, and was probably decisive in its effect upon the final issue of the Thirty Years' War.
In the early spring of 1635, therefore, a French force entered the Netherlands and, after defeating Prince Thomas of Savoy at Namur, joined the Dutch army at Maestricht. Louis XIII had given instructions to the French commanders, Chatillon and de Breze, to place themselves under the orders of the Prince of Orange; and Frederick Henry at the head of 32,000 foot and 9000 horse now entered the enemy's territory and advanced to the neighbourhood of Louvain. Here however, owing to the outbreak of disease among his troops, to lack of supplies and to differences of opinion with his French colleagues, the prince determined to retreat. His action was attended by serious results. His adversary, the Cardinal Infante Ferdinand, was a wary and skilful general. He now seized his opportunity, rapidly made himself master of Diest, Gennep, Goch and Limburg, and took by surprise the important fort of Schenck at the junction of the Waal and the Rhine. Vexed at the loss of a stronghold which guarded two of the main waterways of the land, the stadholder at once laid siege to Schenck. But the Spanish garrison held out obstinately all through the winter and did not surrender until April 26,1636. The Dutch army had suffered much from exposure and sickness during this long investment and was compelled to abstain for some months from active operations. Ferdinand thereupon, as soon as he saw that there was no immediate danger of an attack from the north, resolved to avenge himself upon the French for the part they had taken in the preceding year's campaign. Reinforced by a body of Imperialist troops under Piccolomini he entered France and laid the country waste almost to the gates of Paris. This bold stroke completely frustrated any plans that the allies may have formed for combined action in the late summer.