The first years of the truce were for the United Provinces, now recognised as "free and independent States," a period of remarkable energy and enterprise. The young republic started on its new career with the buoyant hopefulness that comes from the proud consciousness of suffering and dangers bravely met and overcome, and, under the wise and experienced guidance of Oldenbarneveldt, acquired speedily a position and a weight in the Councils of Europe out of all proportion to its geographical area or the numbers of its population. The far-seeing statecraft and practised diplomatic skill of the Advocate never rendered greater services to his country than during these last years of his long tenure of power. A difficult question as to the succession to the Juelich-Cleves duchies arose at the very time of the signing of the truce, which called for delicate and wary treatment.

In March, 1609, the Duke of Juelich and Cleves died without leaving a male heir, and the succession to these important border territories on the Lower Rhine became speedily a burning question. The two principal claimants through the female line were the Elector of Brandenburg and William, Count-Palatine of Neuburg. The Emperor Rudolph II, however, under the pretext of appointing imperial commissioners to adjudicate upon the rival claims, aroused the suspicions of Brandenburg and Neuburg; and these two came to an agreement to enter into joint possession of the duchies, and were styled "the possessors." The Protestant Union at Heidelberg recognised "the possessors," for it was all-important for the balance of power in Germany that these lands should not pass into the hands of a Catholic ruler of the House of Austria. For the same reason Brandenburg and Neuburg were recognised by the States-General, who did not wish to see a partisan of Spain established on their borders. The emperor on his part not only refused to acknowledge "the possessors," but he also sent his cousin Archduke Leopold, Bishop of Passau, to intervene by armed force. Leopold seized the fortress of Juelich and proceeded to establish himself.

It was an awkward situation, for neither the United Provinces nor the archdukes nor the King of Spain had the smallest desire to make the Juelich succession the cause of a renewal of hostilities, immediately after the conclusion of the truce. The eagerness of the French king to precipitate hostilities with the Habsburg powers however forced their hands. Henry IV had for some time been making preparations for war, and he was at the moment irritated by the protection given by the archdukes to the runaway Princess of Conde, who had fled to Brussels. He had succeeded in persuading the States to send an auxiliary force into Germany to assist the French army of invasion in the spring of 1610, when just as the king was on the point of leaving Paris to go to the front he was assassinated on May 14. This event put an end to the expedition, for the regent, Marie de' Medici, was friendly to Austria. The States nevertheless did not feel disposed to leave Leopold in possession of Juelich. Maurice led an army into the duchy and laid siege to the town. It capitulated on September 1. As might have been anticipated, however, the joint rule of the "possessors" did not turn out a success. They quarrelled, and Neuburg asked for Catholic help. Maurice and Spinola in 1614 found themselves again face to face at the head of rival forces, but actual hostilities were avoided; and by the treaty of Nanten (November 12, 1614) it was arranged that the disputed territory should be divided, Brandenburg ruling at Cleves, Neuburg at Juelich. Thus, in the settlement of this thorny question, the influence of Oldenbarneveldt worked for a temporary solution satisfactory to the interests of the United Provinces; nor was his successful intervention in the Juelich-Cleves affair an isolated instance of his diplomatic activity. On the contrary it was almost ubiquitous.

The growth of the Dutch trade in the Baltic had for some years been advancing by leaps and bounds, and now far exceeded that of their old rivals, the Hanseatic league. Christian IV, the ambitious and warlike King of Denmark, had been seriously interfering with this trade by imposing such heavy dues for the passage of the Sound as on the one hand to furnish him with a large revenue, and on the other hand to support his claim to sovereign rights over all traffic with the inland sea. The Hanse towns protested strongly and sought the support of the States-General in actively opposing the Danish king. It was granted. A force of 7000 men under Frederick Henry was sent into Germany to the relief of Brunswick, which was besieged by Christian IV. The siege was raised; and an alliance was concluded between the republic and the Hanse towns for common action in the protection of their commercial interests. Nor was this all. Oldenbarneveldt entered into diplomatic relations with Charles IX of Sweden and with Russia. Cornelis Haga was sent to Stockholm; and from this time forward a close intimacy was established between Sweden and the States. The seaport of Gotheborg, just outside the entrance to the Sound, was founded by a body of Dutch colonists under a certain Abraham Cabelliau, an Amsterdam merchant, and continued to be for years practically a Dutch town.