With little resistance, those Romans eventually conquered the Netherlands; building military outposts and cities, including those at Maastricht and Ultrecht.
Over time, three distinct cultures formed and prospered here: the Franks in the south, the Frisians along the coastlines and the Saxons in the east. By the mid 700's, most of the people were converted to Christianity, and then, the Vikings arrived.
This mostly tranquil expanse of western Europe was first invaded in the early 9th century. The overpowering Vikings removed the accumulated wealth, destroyed some settlements of note, and remained in power until crushed at Ultrecht by a German king in 920. As a result, and beginning with King Otto the Great, German kings held sway here during the 10th and 11th centuries.
Around 1100, coastal swamp lands (all frequently flooded) were being drained and cultivated by the Frisians, farms were developed, towns quickly grew in influence and economic trade groups (leagues) formed.
Repetitive and devastating flooding of the lowlands continued to occur, and over 120,000 people (collectively) drowned in the All Saints Flood (1170), Noordholland Flood (1212), St. Marcellus flood (1219), and the Zuider Zee seawall collapse in year 1287.
Regardless, regional port cities grew powerful, and some became (local) independent empires; nobles and self-appointed rulers converted their own holdings into personal kingdoms, and into that mix (almost predictably) neighbors battled neighbors, wars were fought for land control, and then, a new opportunity presented itself.
With support from local (now tired of bickering) power brokers in the Netherlands, the Flemish Duke of Burgandy (from France), almost by invitation, united all factions; wars were ended and peace and prosperity followed. One of the by-products of that unity was Dutch shipping, as its fleet grew into a consequential force in the 15th century, with Amsterdam the principal port.
Burgundian rule over the "Low Countries" ended in the early 16th century, mostly by conquest. The Habsburg dynasty under Charles V gained control, but in short-order the lengthy struggle for independence surfaced. In 1548, Charles V granted limited autonomy to seventeen provinces of Belgium, Luxembourg and the Netherlands.
When Charles was succeeded by his son, Philip II, the Catholic King of Spain, the new king was outraged by Protestant influence in the "Low Countries," and tried to enforce the brutal Catholic Inquisition policies. Led by Prince William of Orange the locals promptly rebelled, and the resulting Eighty Years' War finally ended in 1648, with the Spanish expelled and independence at hand.