When World War I broke out in August 1914, the Dutch government held to its policy of neutrality, a policy that had applied since the middle of the 19th century. The Dutch army was mobilised and the Defence Line of Amsterdam was manned in addition to other lines and positions. Preparing the 135-kilometre Defence Line of Amsterdam for its defensive task was a huge operation that had to be conducted swiftly according to a pre-defined plan. It seemed like the time had come for the Defence Line of Amsterdam to be put to the test.

During the first few days, mobilised soldiers with the help of civilian workers worked at getting the forts back in shape, organising outposts and partially clearing trees and hedges that obstructed the view from the forts. Meanwhile, inundation staff prepared the inundations, which would only be carried out if an actual siege was imminent.

Although the Netherlands stayed out of World War I, the course of the battle in Belgium and France – especially the prompt capture of the forts at Antwerp – provided new insights with respect to the existing defence works. The forts were considered obsolete in new warfare, and military attention was subsequently aimed more at the army's defence of the country's borders. Nevertheless, large parts of the Defence Line of Amsterdam remained in a state of defence until November 1918.

Fort at Nigtevecht, 1915. The troops falling in line in the main corridor (central postern).

Being billeted anywhere from a few months to two years in the small isolated fort communities was far from appealing. Guard duty, weapon maintenance, military exercises from time to time, digging trenches or the odd short leave did little to change this. There was usually little entertainment at the forts; soldiers played cards, hobby clubs were organised, and drama and music groups were formed. Soldiers could also take language lessons, although the courses were based more on collecting knowledge than providing real education. In short, the fort's greatest enemy was boredom.