From 10,000 to 1500 B.C.E., the population of present-day Denmark evolved from a society of hunters and fishers into an agricultural one. Called Jutland by the end of the 8th century, its mariners belonged to the Vikings, or Norsemen, who raided western Europe and the British Isles from the 9th to 11th century.

The country was Christianized by Saint Ansgar and Harald Blaatand (Bluetooth)—the first Christian king—in the 10th century. Harald's son, Sweyn, conquered England in 1013. His son, Canute the Great, who reigned from 1014 to 1035, united Denmark, England, and Norway under his rule; the southern tip of Sweden was part of Denmark until the 17th century. On Canute's death, civil war tore apart the country until Waldemar I (1157–82) reestablished Danish hegemony over the north.

In 1282, the nobles won the Great Charter, and Eric V was forced to share power with Parliament and a Council of Nobles. Waldemar IV (1340–75) restored Danish power, checked only by the Hanseatic League of north German cities allied with ports from Holland to Poland. His daughter, Margrethe, in 1397 Denmark, Norway, and Sweden united under her rule. But Sweden later achieved autonomy and in 1523, under Gustavus I, independence.

Denmark supported Napoléon, for which it was punished at the Congress of Vienna in 1815 by the loss of Norway to Sweden.

In 1940, Denmark was invaded by the Nazis. King Christian X reluctantly cautioned his fellow Danes to accept the occupation, but there was widespread resistance against the Nazis. Denmark was the only occupied country in World War II to save all its Jews from extermination, by smuggling them out of the country.

Beginning in 1944, Denmark's relationship with its territories changed substantially. In that year, Iceland declared its independence from Denmark, ending a union that had existed since 1380. In 1948, the Faeroe Islands, which had also belonged to Denmark since 1380, were granted home rule, and in 1953, Greenland officially became a territory of Denmark.