The Siege of Amsterdam (1633-1634), while related to the Ostend War, was part of the ongoing conflict between Spain and the Dutch Republic. Grantville's delegation to Amsterdam had elected to remain in the city, partly as a show of confidence, partly to coordinate the medical assistance that had been arranged, and partly because Amsterdam was a relay point for radio messages.
The Spanish forces besieging the city were led by Don Fernando, with naval forces under Antonio de Oquendo blockading the port. Amsterdam was under Fredrick Hendrik as Prince of Orange and stadtholder, but the city itself was effectively controlled by Gretchen Richter and the city's Committee of Correspondence.
Almost from its start, the siege involved a fair amount of negotiation between the sides. Amsterdam's wealth, like that of the Netherlands in general, depended on trade and commerce. If it were cut off from trade for too long, it would be economically ruined, even if it withstood a prolonged siege. Neither Fernando nor Fredrick Hendrik wanted that. Also, Amsterdam's Wisselbank and its bank money, as well as the Dutch guilder, were cornerstones of the larger European economy. After considerable negotiation, agreements were reached which ensured the Wisselbank's security and allowed it to continue operating, and which allowed raw materials and finished products to pass through the siege lines. Also, radio stations were placed in Amsterdam and Antwerp. While the siege went on into mid-1634, it was not pressed hard, and it become increasingly clear that the siege would eventually be settled by a negotiated political agreement between Fernando and Fredrick Hendrik.
In late May of 1634, Rubens used the impending arrival of the timberclad SSIM Achates and its accompanying ships to persuade Fernando to propose a cease-fire throughout the Netherlands, pointing out that Michael Stearns was deliberately giving him the opportunity to do it without appearing to have been forced into it. This ended the siege, and laid the groundwork for the agreement which established a unified Dutch kingdom.