Denmark remained neutral during World War I and did not take part in the warfare. The biggest event from a Danish perspective is the reunification with Northern Schleswig (Sønderjylland) in 1920. After the Second War of Schleswig in 1864, Denmark was forced to cede Schleswig and Holstein to Prussia. In 1918, the Versailles powers offered to return the region of Schleswig-Holstein to Denmark. After the Schleswig Plebiscites Northern Schleswig (Sønderjylland) was recovered by Denmark in 1920. The reunion day ("Genforeningsdag") is celebrated every 15 June on "Valdemarsdag".