Denmark was also the suzerain of the Slavic Principality of Rügen in modern north-eastern Germany in the High Middle Ages before it passed under the suzerainty of the Holy Roman Empire.

The war ended on 30 October 1864, when the Treaty of Vienna caused Denmark's cession of the Duchies of Schleswig, Holstein, and Saxe-Lauenburg to Prussia and Austria.

The 1914 Septemberprogramm authorized by German Chancellor Theobald von Bethmann Hollweg proposed the creation of a Central European Economic Union, comprising a number of European countries, including Germany and Denmark, in which, as the Chancellor secretly stressed, there was to be a semblance of equality among the member states, but in fact it was to be under German leadership to stabilize Germany's economic predominance in Central Europe, with co-author Kurt Riezler admitting that the union would be a veiled form of German domination in Europe (see also: Mitteleuropa).

Denmark, despite having remained neutral all throughout World War I, still ended up involved in the negotiations following the defeat of Germany, due to US President Woodrow Wilson's Fourteen Points listing the different people of Europe's right to self-determination amongst its principles, and the substantial Danish minority living in the Southern Jutland/Northern Schleswig area.

Eventually, it was decided that the question of national affiliation for the people living in the area would be settled through a series of public plebiscites.

In the early morning of 9 April 1940, Germany invaded Denmark and Norway, ostensibly as a preventive maneuver against a planned, and openly discussed, Franco-British occupation of both these countries.

After the invasions, envoys of the Germans informed the governments of Denmark and Norway that the Wehrmacht had come to protect the countries' neutrality against Franco-British aggression.

Given Denmark's position in relation to the Baltic Sea the country was also important for the control of naval and shipping access to major German and Soviet harbours.

Nevertheless, in the early morning hours, a few Danish troops engaged the German army, suffering losses of 16 dead and 20 wounded.

[13] At 04:00 on 9 April 1940, the German ambassador to Denmark, Renthe-Fink, called the Danish Foreign Minister Munch and requested a meeting with him.

When the two men met 20 minutes later, Renthe-Fink declared that German troops were at that moment moving in to occupy Denmark to protect the country from Franco-British attack.

German Fallschirmjäger units had made unopposed landings and taken two airfields at Aalborg, the Storstrøm Bridge as well as the fortress of Masnedø.

[13] At 04:20 local time, 1,000 German infantrymen landed in Copenhagen harbour from the minelayer Hansestadt Danzig, quickly capturing the Danish garrison at the Citadel without encountering resistance.

The first German attack on Amalienborg was repulsed, giving Christian X and his ministers time to confer with the Danish Army chief General Prior.

Faced with the explicit threat of the Luftwaffe bombing the civilian population of Copenhagen, and only General Prior in favour of continuing to fight, King Christian X and the entire Danish government capitulated at 08:34 in exchange for retaining political independence in domestic matters.

[13] At 05:45, two squadrons of German Bf 110s attacked Værløse airfield on Zealand and wiped out the Danish Army Air Service by strafing.