Operation Weserübung - or Weser Exercise, after the German Weser River - began in the early hours of April 9th 1940, 75 years ago.
Germany had already conquered Poland as the Second World War started and Adolf Hitler's Nazi forces set their sights on bordering country Denmark, and Norway, strategic locations that the British military had also been eyeing.
The operation sought to preempt the British moving into the area. Denmark's location on the Baltic Sea with its proximity to Soviet harbours also made it important to control.
German troops arrived in the southern island town of Gedser by ferry at about 4am and continued north while German authorities threatened to bomb Copenhagen if Denmark resisted.
Danish soldiers on the morning of the German invasion. Photo: Quibik / Wikimedia Commons.
King Christian X surrendered at about 6am - just two hours after German troops arrived, making it the shortest German military campaign during the war.
In Norway, on the other hand, it took 62 days for the country to be brought under full German control, distinguishing it as the nation in Western Europe which held out the longest in the face of Nazi attack.
Reigning Danish Queen Margrethe II was born just a week after the invasion on April 16th 1940.
Immediately after the invasion, Denmark was able to keep its government and monarchy in place with a mix of democracy and totalitarianism until Germany dissolved the government in 1943 following strikes and upheaval.
Although some 6,000 Danes are estimated to have joined the corps of Danish Nazis known as Free Corps Denmark during the war, the Danish resistance movement is estimated to have included well over 20,000 Danes who worked to actively undermine the German occupation.
Danish fishermen also put themselves at great risk by ferrying Denmark’s Jews to safety in Sweden.
Denmark publicly apologized for collaboration with the Nazis for the first time in 2003. Prime Minister Anders Fogh Rasmussen called the co-operation "morally unjustifiable" in a speech for the 60th anniversary of the end of the 1940 to 1943 collaborationist government.
A ceremony was set to take place at the Danish Army headquarters Søgårdlejren in Aabenraa, where Crown Princess Mary would be in attendance to remember the 16 Danes who died in the invasion.
The military also planned events at the old barracks in Haderslev and at the old airport in Værløse.
Scandinavian public service TV stations have worked together on a documentary project about Hitler’s influence on the Scandinavian countries during the Second World War. The first episode in the five-part series was shown on DR K on April 8th.