 In the 1920s, when the Schleswig Plebiscite incorporated Northern Schleswig into Denmark, 76.5% of Tønder's inhabitants voted to remain part of Germany and 23.5% voted to join Denmark.During World War I, a Zeppelin base was operated in Tønder by the Imperial German Navy.The base was attacked by the British on 19 July 1918, in what is known as the Tondern raid.