Warmatrix

The Eighty Years' War, lasting from 1568 CE to 1648 CE, was the struggle of the Dutch to gain independence from the western Habsburg empire, ruled by Philip II of Spain. It was a long conflict that not only involved the Dutch and Spanish, but also the Flemish, English, Germans and French. Economy played a larger role than military factors, both in causing and fighting it.
In the first half of the 16th century CE the Habsburg empire had gained sovereignty over the Low Countries. It used them as a cash cow to finance its wars against France and the Ottoman empire, which cost the inhabitants a lot but gained them nothing. When the protestant religious movement that had gained influence in the north was repressed by the Habsburgs, the Dutch ravaged catholic churches in 1566 CE. Philip II of Spain, ruler of the western half of the Habsburg empire, sent Fernando Alvarez de Toledo, duke of Alba, north with an army of 9,000 men to quell the troubles. However he achieved the opposite: his taxes and savage persecutions led to armed resistance and finally a full revolt in 1572 CE.
The Dutch resistance started as a hodgepodge of calvinist fanatics; "Watergeuzen" (Sea Beggars), privateers / pirates; mercenaries from England and Germany, led by nobles like William the Silent. They were no match for Alba's seasoned Tercios and evaded pitched battles. But the densely populated, water-rich country required more than dominance on the battlefield to control. When the Watergeuzen captured the port of Brill, many, though not all, important cities went over to the rebels. Some had been fortified as star fortresses and many more would be upgraded to such during the war. Alba struck back mercilessly, but was forced to take them one by one in extensive sieges. The Dutch often used inundation to frustrate the attacks of the Spanish.
In 1575 CE Alba ran out of money and his troops became mutinous. He had to pull back to the southern half of the Low Countries. The provinces of Holland and Sealand created a proto-state, the Union of Utrecht, while the southern ones tried to re-establish their Medieval position of dominance. The northern provinces joined with the Union of Utrecht. In 1581 CE the Republic of the Seven United Netherlands was proclaimed. In the meanwhile the Spanish financial situation stabilized and Alba was replaced by Alexander Farnese, duke of Parma. By 1582 CE he had 61,000 men under his command and used them to regain territory, almost bringing the fledgeling republic to its knees.
The Dutch did not try to become independent right away. Instead they tried to get the French king and later English queen to become their sovereigns, though these rulers were too afraid of Spain's might to do so. The English earl of Leicester ruled the north for a few years as governor-general, but achieved little and was more or less tossed out after a few years.
Spain's military might was seriously dented in 1588 CE with the defeat of its armada. In the same year the Dutch reorganized their economy and got a boost from skilled immigrants from Flanders. The wealth was used to increase the size of the army from 20,000 to 32,000 men. Maurice of Nassau became head of the army and proved himself to be a capable siege engineer and military innovator. He reformed the army from an amateur rabble to a well-paid, superbly disciplined force, introduced line infantry tactics and went to the offensive. In one decade the Dutch regained much lost ground. But when probing attacks into Flanders met with little success and a few attacks by the new Spanish commander Spinola showed that the republic was still vulnerable, the fighting ran into a stalemate. In 1609 CE the Twelve Year's Truce was signed.
The truce should have paved the way to a peace settlement, but the Dutch then, as throughout the war, were too divided. Holland sometimes dominated the other provinces and sometimes had to make concessions; the stadholders tried to wrest power from the States-General; north and south had different aims and the religious factions fought each other without any compromise. The war was very much an economic one, where different parties had different interests. The Dutch even sold weapons to the Spanish and used the profits to finance their own army.
The truce made the republic to be recognized as an independent state. During it, the Dutch economic focus shifted away from America to northern Europe. Both army and navy were built up to a point where they could rival their Spanish counterparts in the northern theater. Maurice drew more and more power to himself and got the republic involved in the Thirty Years' War, mainly financially and politically. In 1621 CE the Twelve Year's Truce was not renewed. Both parties stepped up their military expenditure and the whole of the republic was under siege for 8 years. It was protected by a ring of fortresses, which were probed by Spinola and his army. However the Spanish had already abandoned hope of reconquering the lands in the north. Instead they targeted Dutch shipping in the Atlantic. The Dunkirk pirates, financed by Spain, hurt the Dutch trade severely. The Dutch hit back, mostly targeting Portugal's colonies because the Spanish ones were too strong. Both parties lost, the Dutch the most; England and France profited and gained footholds in the Caribbean.
After 1629 CE, with Spain making the mistake of involving itself in the War of Mantuan Succession, the republic again had the upper hand. Under Frederick Henry, brother of Maurice, new offensives were launched. In 1635 CE he made the republic join up with France to squeeze the southern Netherlands. Like his brother, he drew much power to himself and his clique. But the Spanish cardinal-infante Olivares skillfully held off the French and counterattacked against the Dutch, forcing both to back down. In the years after 1640 CE neither side made much progress. Finally all Dutch parties involved reached a consensus and made peace with Spain in 1648 CE, the Peace of Münster, where the independence of the republic was formally recognized.
The republic went on to fight many other wars, juggling a balance between great European powers like France and England, sometimes victorious, sometimes on the brink of disaster. As a state it lasted until 1795 CE, but as a nation until this day.