 By the mid-17th century the Netherlands was the world's leading economic and maritime power, a period of economic, scientific and cultural growth later known as the Dutch Golden Age.Despite these successes, the Dutch failed to expel the Spanish from the wealthy provinces of the Southern Netherlands, modern-day Belgium, Luxembourg plus the Nord-Pas-de-Calais and Longwy regions of northern France.France had financed the Dutch for many years to weaken the Habsburgs, then the dominant power in Europe; after the Treaty of Prague in 1635 it formally declared war on Spain and the Holy Roman Empire.