The Grebbe Line (Grebbelinie) was a Dutch defence line that ran from the Grebbeberg in Rhenen to the former Zuiderzee. The line was based on inundation. If an enemy invasion was imminent, large areas 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 wasn’t needed anymore and it was finally decommissioned in 1926. International tensions grew in the late 1930s and the Dutch army had to prepare for an enemy invasion. In 1940 the decision was made to make the Grebbe Line the main defensive line again. The old fortifications were extended with trenches, roadblocks and casemates. On May 10, 1940 the German army invaded The Netherlands. Different parts of the Grebbe 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 had to surrender after the bombing of Rotterdam. After the war the Grebbelinie was permanently decommissioned.