Knowledge of Danish antiquity is derived largely from archaeological research. Some historians believe that Danes inhabiting the southern part of the Scandinavian Peninsula migrated to the Jutland Peninsula and the adjacent islands in the Baltic Sea in the 5th and 6th centuries. Evidence of major public structures—including a canal, a long bridge, and the ramparts across the neck of Jutland now called the Danevirke—in the 8th century attests to the presence of a fairly strong central authority in Jutland on the eve of the Viking age. Within a century of their first raid on the British Isles in the 780s, the Danes were masters of the part of England that became known as the Danelaw. Under King Harold Bluetooth in the 10th century, political consolidation increased, and the Christianization of the Danes was begun. Harold’s son, Sweyn I, conquered all of England in 1013 and 1014. Sweyn’s son, Canute II, who ruled England (1016-1035) and Denmark (1018-1035), completed the Christianization of Denmark.

After 500 BC, bronze was gradually replaced by iron, and a village society developed in a landscape of bogs, meadows, and woods with large clearings. The villages appear to have been moved and the fields abandoned with each new generation. Chiefs and rich farmers lived in houses between 40 and 100 feet in length, the climate now being colder and wetter; as in the Bronze Age, objects of great value were laid as offerings in the bogs. The period up to AD 400 was marked by the large number of villages, and splendidly equipped graves suggest that political power was gathered in fewer hands. More or less fixed trading connections were established with the Romans; about AD 200 the first runic inscription appeared, possibly developed under the influence of the Latin alphabet. The period from 400 to 800 is known as the Germanic Iron Age, but the finds have been few, indicating a time of decline, unrest, and bubonic plague in the 6th century. The first trading markets appeared at Hedeby (near what is now Schleswig, Ger.) and Ribe in the 8th century, and written sources mention the existence of slaves.

In 1380 Denmark and Norway were joined in a union under one king, Olaf II, a grandson of Waldemar IV, and with Norway came Iceland and the Faroe Islands. After Olaf’s death in 1387, his mother, Margaret I, reigned in his stead. In 1389 she obtained the crown of Sweden and began the struggle, completed successfully in 1397, to form the Union of Kalmar, a political union of the three realms. Denmark was the dominant power, but Swedish aristocrats strove repeatedly—and with some success—for Sweden’s autonomy within the union. The Kalmar Union lasted until 1523, when Sweden won its independence in a revolt against the tyrannical Christian II led by Gustav Vasa, who was elected king of Sweden as Gustav I in that year.