Dutch Waterline

The Dutch Waterline (Dutch: Hollandsche Waterlinie; modern spelling: Hollandse Waterlinie) was a series of water-based defences conceived by Maurice of Nassau in the early 17th century, and realised by his half brother Frederick Henry. Combined with natural bodies of water, the Waterline could be used to transform Holland, the westernmost region of the Netherlands and adjacent to the North Sea, almost into an island. In the 19th century, the Line was extended to include Utrecht.

On July 26, 2021, the line was added to the Defence Line of Amsterdam to become the Dutch Water Defence Lines UNESCO World Heritage Site.

Early in the Eighty Years' War of Independence against Spain, the Dutch realized that flooding low-lying areas formed an excellent defence against enemy troops. This was demonstrated, for example, during the Siege of Leiden in 1574. In the latter half of the war, when the province of Holland had been freed of Spanish troops, Maurice of Nassau planned to defend it with a line of flooded land protected by fortresses that ran from the Zuiderzee (present IJsselmeer) down to the river Waal.

In 1629, Prince Frederick Henry started the execution of the plan. Sluices were constructed in dikes and forts and fortified towns were created at strategic points along the line with guns covering the dikes that traversed the water line. The water level in the flooded areas was carefully maintained at a level deep enough to make an advance on foot precarious and shallow enough to rule out effective use of boats (other than the flat bottomed gun barges used by the Dutch defenders). Under the water level additional obstacles like ditches and trous de loup (and much later, barbed wire and land mines) were hidden. The trees lining the dikes that formed the only roads through the line could be turned into abatis in time of war. In wintertime the water level could be manipulated to weaken ice covering, while the ice itself could be used when broken up to form further obstacles that would expose advancing troops to fire from the defenders for longer.

The Dutch Water Line proved its value less than forty years after its construction during the Franco-Dutch War (or Third Anglo-Dutch War) (1672), when it stopped the armies of Louis XIV from conquering Holland, although the freezing over of the line came close to rendering it useless. In 1794 and 1795, the revolutionary French armies overcame the obstacle posed by the Dutch Water Line only by the heavy frost that had frozen the flooded areas solid.

After the final defeat of Napoleon in 1815 at the Battle of Waterloo, the United Kingdom of the Netherlands was formed. Soon after King William I decided to modernise the Water Line. The Water Line was partly shifted east of Utrecht.

In the next 100 years the main Dutch defence line would be the new Water Line. It was further extended and modernised in the 19th century, with forts containing round gun towers reminiscent of Martello towers. The line was mobilised but never attacked during the Franco-Prussian War in 1870 and World War I.

At the advent of World War II, most of the earth and brick fortifications in the Water Line were too vulnerable to modern artillery and bombs to withstand a protracted siege. To remedy this a large number of pillboxes were added. However, the Dutch had decided to use a more eastern main defence line, the Grebbe Line, and reserved a secondary role for the Water Line.