On the abolition of the Holy Roman Empire in 1806, Holstein was informally incorporated in Denmark. This was later reversed, and while Schleswig remained as before, Holstein and Lauenburg were included in the new German Confederation. After Germany lost World War One, in which Denmark had been neutral, the victors continued to punish Germany by offering Denmark the opportunity to redraw the border between Denmark and Germany.

Although Denmark had not participated in the War, the vindictive Allied powers at Versailles arranged a referendum in Northern and Central Schleswig (the Prussian Province of Schleswig-Holstein, the former Duchy of Schleswig). In Northern Schleswig on February 10, 1920 75% voted for re-unification with Denmark and 25% voted for Germany. In Central Schleswig on March 14, 1920 the results were reversed; 80% voted for Germany and just 20% for Denmark, primarily in Flensburg. No vote ever took place in the southern third of Schleswig, because the result for Germany was predictable. On June 15, 1920, Northern Schleswig officially returned to Danish rule. Germany continued to hold the whole of Holstein and southern and central Schleswig, later becoming the Prussian province of Schleswig-Holstein. The Danish-German border imposed on Germany following World War One was not later challenged by Germany, yet after Germany lost World War Two, the victors again instigated Denmark to reacquire some of its lost territory in Schleswig.

The German National Monument, the Düppeldenkmal, was a reminder of the centuries old German population in North Schleswig. In May, 1945 the monument was blown up by the Danes, as was the Arnkiel Monument a month later. In August of that year, the Knivsbergturm, the largest remaining German National Monument in Denmark, was blown up in North Schleswig.