The Grebbe Line (Grebbelinie) was an old Dutch defence line that ran from the Grebbeberg in Rhenen to the former Zuiderzee. The line was based on inundation, so when an enemy invasion was imminent, large areas of land could be flooded. The line was first established in 1745 but it was never really used. In 1815 the construction of the New Dutch Water Line started. The Grebbe Line became obsolete and it was decommissioned in 1926.
International tensions grew in the late 1930s. Although The Netherlands followed a strict policy of neutrality, the Dutch army had to prepare for a possible enemy invasion. Early 1940 the decision was made to make the Grebbe Line the main defensive line again. The old fortifications were now extended with trenches, roadblocks and casemates.
On May 10, 1940 the German army invaded The Netherlands. Different parts of the line were attacked but the most dramatic fighting took place on the Grebbeberg. The German army broke through the line here on May 13. The Dutch army retreated behind the New Dutch Water Line but surrendered after the bombing of the city of Rotterdam.