The years 1621–1648 constituted the final phase of the Eighty Years' War (c. 1568–1648) between the Spanish Empire and the emerging Dutch Republic. It began when the Twelve Years' Truce (1609–1621) expired, and concluded with the Peace of Münster in 1648.
Although the Dutch and Spanish were both involved in opposite sides of the War of the Jülich Succession (June 1609 – October 1610; May–October 1614) in Jülich-Cleves-Berg, they carefully avoided each other, and thus the hostilities never spread back into the Habsburg Netherlands, and the truce held firm. Nevertheless, attempts to negotiate a definitive peace also failed, and the war resumed as anticipated in 1621. Essentially, it became a side theatre of the wider Thirty Years' War that had already broken out with the Bohemian Revolt in 1618 in eastern parts of the Holy Roman Empire (Bohemia and Austria), pitting Central Europe's Protestant Union against the Catholic League, although the two conflicts never fully merged. With several back and forths – notably, the Spanish conquered Breda in 1625, but the Dutch took it back in 1637 – the Dutch Republic was able to conquer the eastern border forts of Oldenzaal (1626) and Groenlo (1627), the major Brabantian city of 's-Hertogenbosch (1629), the fortified cities of Venlo, Roermond and Maastricht along the Meuse (1632), and Sas van Gent (1644) and Hulst (1645) in Zeelandic Flanders.
Nevertheless, peace talks in 1629–1630 came to nothing. More ambitious plans to conquer Brussels in 1632–1633 with the help of anti-Spanish nobility in the Southern Netherlands never came to fruition. Several attempted Northern republican surprises and sieges of Antwerp were parried by the Spanish Army of Flanders.
Nor did the Franco-Dutch alliance bring significant changes to the situation on the ground. It began with a disastrous Franco-Dutch invasion of the southern Netherlands in 1635. This in fact made matters worse for the Dutch when French and Dutch troops sacked the city of Tienen, which cost them the sympathies of the southern Netherlands population.
However, French intervention and internal discontent at the costs of the war in the Low Countries led to a change in Spain's 'Netherlands First' policy. Instead Spain focused on suppressing the French-backed Reapers' War in Catalonia. The resulting stalemate and financial troubles, plus Spanish military exhaustion and Dutch desire for formal political recognition, eventually convinced both sides in the mid-1640s to hold peace talks.
The outcome was the 1648 Peace of Münster, which confirmed most agreements already reached with the Truce of 1609.
Background
Van Oldenbarnevelt had no ambition to have the Republic become the leading power of Protestant Europe, and he had shown restraint when, in 1609–1610 and 1614, the Republic had felt constrained to intervene militarily in the Jülich-Cleves crisis opposite Spain. Though there had been a danger of armed conflict between the Spanish and Dutch forces involved in the crisis, both sides took care to avoid each other, respecting each other's spheres of influence.
Negotiations for a permanent peace went on throughout the truce. Two major issues could not be resolved. First, the Spanish demand for religious freedom of Catholics in the Dutch Republic was countered by a Dutch demand for a similar religious freedom for Protestants in the Southern Netherlands. Second, there was a growing disagreement over the trade routes to the different colonies (in the Far East and the Americas). The Dutch used their navy to enlarge their colonial trade routes to the detriment of Spain (primarily concentrating on capturing Philip's Portuguese possessions, since Portugal had not signed the truce). On behalf of the Austrian archdukes, Brabantian diplomat Petrus Peckius the Younger made one last attempt at negotiating a renewal of the truce in March 1621 in The Hague. He addressed the States-General on 23 March 1621, proposing that the Dutch Republic would be left to run its own affairs in return for a nominal recognition of the sovereignty of the king of Spain. This suggestion was not well received by his hosts and he was sent back with an indignant rejection of his proposal.Cf. Valerius Gedenckclanck, vol 2, pp. 224ff The war was on once more – and crucially, at risk of fully merging with the wider Thirty Years' War that had already broken out in 1618.
Dutch intervention in the early stages of the Thirty Years' War (1619–1621)
While the Twelve Years' Truce was still underway, civil war had begun in the Kingdom of Bohemia following the Second defenestration of Prague on 23 May 1618. The Bohemian insurgents were now pitted against their king, Ferdinand, who would soon succeed his uncle Matthias (the former States General governor-general of the Netherlands) as Holy Roman Emperor. They cast about for support in this struggle, and on the Protestant side only the Republic was able and willing to provide it. This took the form of support for Frederick V, Elector Palatine, a nephew of Prince MauriceFrederick was the son of Maurice's half-sister Countess Louise Juliana of Nassau and a son-in-law of James I, when Frederick accepted the crown of Bohemia the insurgents offered him (he was crowned on 4 November 1619). His father-in-law had sought to restrain him from doing this, warning that he could not count on English aid, but Maurice encouraged him in every way, providing a large subsidy and promising Dutch armed assistance. The Dutch therefore played a large role in precipitating the Thirty Years' War.
Maurice's motivation was the desire to manoeuvre the Republic into a better position should the war with Spain resume after the expiration of the truce in 1621. Renewal of the truce was a distinct possibility, but it had become less likely, as both in Spain and in the Republic more hard-line factions had come to power.The Duke of Lerma had been replaced by his son in 1618 Though civil war had been avoided in the Republic, national unity had been bought with much bitterness on the losing Remonstrant side, and Maurice for the moment had to garrison several former Remonstrant-dominated cities to guard against insurrection. This encouraged the Spanish government, perceiving internal weakness in the Republic, to choose a bolder policy in the Bohemian question than they otherwise might have done. The Bohemian war therefore soon degenerated into a proxy war between Spain and the Republic. Even after the Battle of White Mountain of November 1620, which ended disastrously for the Protestant army (one-eighth of which was in the Dutch pay), the Dutch continued to support Frederick militarily, both in Bohemia and in the Electoral Palatinate. Maurice also provided diplomatic support, pressing both the Protestant German princes and James I to come to Frederick's aid. When James sent 4,000 English troops in September 1620, these were armed and transported by the Dutch, and their advance covered by a Dutch cavalry column.
In the end the Dutch intervention was in vain. After just a few months, Frederick and his wife Elizabeth fled into exile at The Hague, where they became known as the Winter King and Queen for their brief reign. Maurice pressed Frederick, in vain, to at least defend the Palatinate against the Spanish troops under Spinola and Tilly. This stage of the war witnessed the triumph of Spain and the Imperial forces in Germany. James held this against Maurice for his incitement of the losing side with promises that he could not keep.
There was continual contact between Maurice and the government in Brussels during 1620 and 1621 regarding a possible renewal of the truce. Archduke Albert of Austria, who had first become Governor General of the Habsburg Netherlands, then its sovereign (following his marriage to Isabella Clara Eugenia, the daughter of Philip II), was in favour of a renewal, especially after Maurice falsely gave him the impression that a peace would be possible on the basis of a token recognition by the Republic of the sovereignty of the king of Spain. When Albert sent the chancellor of Brabant, Petrus Peckius, to The Hague to negotiate with the States General on this basis, he fell into this trap and mentioned this recognition, instantly alienating his hosts. Nothing was as certain to unite the northern provinces as the suggestion that they should abandon their hard-fought sovereignty. If this incident had not come up, the negotiations might well have been successful as a number of the provinces were amenable to simply renewing the truce on its existing terms. Now formal negotiations were broken off, however, and Maurice was authorised to conduct further negotiations in secret. His attempts to get a better deal met with counter-demands from the new Spanish government for more substantive Dutch concessions. The Spaniards demanded Dutch evacuation of the West and East Indies; lifting of the restrictions on Antwerp's trade by way of the Scheldt; and toleration of the public practice of the Catholic religion in the Republic. These demands were unacceptable to Maurice and the truce expired in April 1621.
The war did not immediately resume, however. Maurice continued sending secret offers to Isabella after Albert had died in July 1621,Under the terms of the Will of Philip II the pretended sovereignty over the Netherlands now reverted to Philip III. through the intermediary of the Flemish painter and diplomat Peter Paul Rubens. Though the contents of these offers (which amounted to a version of the concessions demanded by Spain) were not known in the Republic, the fact of the secret negotiations became known. Proponents of restarting the war were disquieted, like the investors in the Dutch West India Company, which after a long delay was finally about to be founded, with a main objective of bringing the war to Spanish America. Opposition against the peace feelers therefore mounted, and nothing came of them.
The Republic under siege (1621–1628)
Spanish power transfer and Spinola's conquest of Breda (1621–1625) The truce officially expired on 9 April 1621; Philip III had died shortly before, on 31 March. He was succeeded by his 16-year-old son Philip IV, and the new government under Gaspar de Guzmán, Count-Duke of Olivares had to get settled. Shortly after Albert of Austria also died on 13 July, so that Austrian Habsburg rule over the (Southern) Netherlands reverted to the Spanish branch. Isabella Clara Eugenia lost her sovereignty over the Netherlands upon her husband's death, and became governor-general for Philip IV instead.