The Kalmar Union is a historiographical term meaning a series of personal unions that united the three kingdoms of Denmark, Norway (with Iceland, Greenland, Faroe Islands, Shetland, and Orkney), and Sweden (including Finland) under a single monarch, though intermittently and with a population less than 3,000,000.

[3] The countries had not technically given up their sovereignty, nor their independence, but in practical terms, they were not autonomous, the common monarch holding the sovereignty and, particularly, leading foreign policy; diverging interests (especially the Swedish nobility's dissatisfaction over the dominant role played by Denmark and Holstein) gave rise to a conflict that would hamper the union in several intervals from the 1430s until the union's breakup in 1523 when Gustav Vasa became king of Sweden.

Norway and its overseas dependencies, however, continued to remain a part of the realm of Denmark–Norway under the Oldenburg dynasty for several centuries until its dissolution in 1814.

Denmark–Norway is the historiographical name for a former political entity consisting of the kingdoms of Denmark and Norway, including the originally Norwegian dependencies of Iceland, Greenland and the Faroe Islands.

Both Denmark and Norway were invaded by Germany in 1940, and the mainland territories of both countries were afterwards under German occupation with relatively light military casualties.