How losing Antwerp split the Low Countries and helped make Amsterdam rich
Spain let about 60,000 Protestants walk out, and the bankers among them carried their money and trade to Amsterdam.

The Rest Is History · The Shadow War: Elizabeth I vs The Catholics (Part 4)On August 17, 1585, after a siege lasting 13 months, Antwerp surrendered to the Spanish. It was then the biggest and richest city in the Low Countries, and its fall drew the line that still separates the Netherlands from Belgium.
The mechanism was geography plus religion. Four great rivers cut across the Low Countries and formed a natural barrier. North of those rivers, the rebel provinces could hold out. That north would become the Dutch Republic, a Protestant powerhouse of the 1600s, madly rich, and a place that helped create modern capitalism. South of the rivers, the provinces were brought back under Spanish Hapsburg rule, returned to Catholicism, and stayed that way until the French Revolution. Centuries later, that south would become Belgium. One shorthand is Dutch painters like Rembrandt on one side and the Antwerp painter Rubens on the other, or the rising port of Amsterdam against the older port of Antwerp.
Antwerp mattered because it paid for the revolt. The city controlled the Scheldt, the river estuary that linked the rich cloth towns of Flanders, like Ghent and Bruges, to the North Sea and to trade with England. People thought its massive fortifications made it impregnable, but Alexander Farnese, the Prince of Parma and Spain's commander in the Netherlands, besieged it from June 1584 with a ring of forts to cut it off from land and sea.
When Parma won, he was generous in victory. As at Ghent, which had fallen the previous September, he restored Catholicism as the official religion but let Protestants leave rather than face persecution. Huge numbers took the offer. At Ghent, about 15,000 people, roughly a third of the city, left for Holland. At Antwerp, probably around 60,000 people out of about 100,000 left.
In plain terms, the city lost its money men and their skills to a rival. Many of those who walked out were bankers and merchants, the people behind its most sophisticated trade and finance. Most headed north to Holland, still the main stronghold of the revolt, and many settled in Amsterdam, then an upstart business town. In this telling, their money, skills, and trading networks helped make Amsterdam the leading commercial center of the 1600s.
In short, Spain won the city and lost the capitalists. Antwerp never recovered its lead, and Amsterdam built the economy we now associate with the Dutch Golden Age.
Questions people ask
Why was control of the Scheldt estuary so important?
The Scheldt is the river that connects Antwerp to the North Sea. Whoever controlled its mouth controlled customs, shipping, and access for Flemish cloth to England and beyond, which is why cutting Antwerp off from the sea strangled the rebellion.
What happened to Antwerp after so many people left?
The Dutch closed the Scheldt to shipping after Antwerp fell, so large ocean trade could not reach Antwerp and was diverted to Amsterdam and other northern ports. Antwerp remained a major city, especially for art under Rubens, but Amsterdam took over as the trading hub for more than a century.
Why did the refugees choose Amsterdam in particular?
Holland was the largest and richest of the seven northern provinces, protected in part by rivers and flooded lowlands that were hard for an army to cross. Amsterdam was its booming port, so refugees with capital and commercial contacts naturally clustered there.