The main cities of Denmark–Norway were Copenhagen, Christiania (Oslo), Altona, Bergen and Trondheim, and the primary official languages were Danish and German, but Norwegian, Icelandic, Faroese, Sami and Greenlandic were also spoken locally.

From 1536/1537, Denmark and Norway formed a personal union that would eventually develop into the 1660 integrated state called Denmark–Norway by modern historians, at the time sometimes referred to as the "Twin Kingdoms".

The Dano-Norwegian union lasted until 1814,[12] when the Treaty of Kiel decreed that Norway (except for the Faroe Islands, Iceland, and Greenland) be ceded to Sweden.

The term "Kingdom of Denmark" is sometimes used to include both countries in the period, since the political and economic power emanated from the Danish capital, Copenhagen.

Centralisation was supported in many parts of Norway, where the two-year attempt by Sweden to control Trøndelag had met strong local resistance and resulted in a complete failure for the Swedes and a devastation of the province.

At the earliest times this meant areas in Northern Europe and North America, for instance Estonia and the Norwegian possessions of Greenland, the Faroe Islands and Iceland.

At its height the empire was about 2,655,564.76 km2 (1,025,319 sq mi),[note 1] after the dissolution of the union, in 1814, all the overseas territories became a part of Denmark.

These tolls made up two thirds of Denmark's state income, and allowed Danish-Norwegian kings such as Christian IV to become extremely rich.

[citation needed] Christian III, who had relied on Swedish aid in the Count's Feud, kept peaceful relations with Sweden throughout his reign.

When Denmark purchased Osel, Duke Magnus, brother of King Frederick II was granted control of the island.

When Frederick II included the traditionally Swedish insignia of three crowns into his own coat of arms, the Swedes interpreted this as a Danish claim over Sweden.

In response, Erik XIV of Sweden (reigned 1560–1568) added the insignia of Norway and Denmark to his own coat of arms.

Swedish king Charles IX's way of accomplishing this was to try to set up a new trade route through Lapland and northern Norway.

In 1607 Charles IX declared himself "King of the Lapps in Nordland", and started collecting taxes in Norwegian territory.

[9][15] With the money provided by the aforementioned states, along with his own personal fortune, Christian could hire a large army of mercenaries.

After another defeat at the Battle of Wolgast and following the Treaty of Lübeck in 1629, which forbade Denmark–Norway from future intervening in German affairs, Denmark–Norways's participation in the war came to an end.

In 1643 the Swedish Privy Council determined that the chances of a gain in territory for Sweden in an eventual war against Denmark–Norway would be good.

[citation needed] Denmark was poorly prepared for the war, and Norway was reluctant to attack Sweden, which left the Swedes in a good position.

Thus the Thirty Years' War facilitated rise of Sweden as a great power, while it marked the start of decline for Denmark–Norway.

In the aftermath of Sweden's final secession from the Kalmar Union in 1521, civil war and the Protestant Reformation followed in Denmark and in Norway.

In 1537, during the Count's Feud, king Christian III of Denmark–Norway staged a coup d'état in Norway, and made it a hereditary kingdom in a real union with Denmark.

These changes were confirmed in the Leges regiae signed on 14 November 1665, stipulating that all power lay in the hands of the king, who was only responsible to God.

Denmark had lost its provinces in Scania after the Treaty of Roskilde and was always eager to retrieve them, but as Sweden had grown into a great power it would not be an easy task.

[citation needed] Although the Danish-Norwegian assault began as a great success, the Swedes led by 19-year-old Charles XI counter-attacked and took back the land that was being occupied.

The Dano-Norwegian navy was caught unprepared for any military operation and the British found their ships still in dock after the winter season.

For many Danish people who had the possibility to leave Denmark proper, such as merchants and civil servants, Norway was seen as an attractive country of opportunities.

In the end, Pietism was never firmly established as a lasting religious grouping, but policies enacted by the "pietist king" affects citizens of Denmark, Norway and Iceland to this day, like the Holiday Peace Act.

[citation needed] Since the late 19th century the Danish–Norwegian union was increasingly viewed in a more nuanced and favourable light in Norway with a stronger focus on empirical research, and historians have highlighted that the Norwegian economy thrived and that Norway was one of the world's wealthiest countries during the entire period of real union with Denmark.

Historians have also pointed out that Norway was a separate state, with its own army, legal system and other institutions, with significant autonomy in its internal affairs, and that it was primarily governed by a local elite of civil servants who identified as Norwegian, albeit in the name of the "Danish" King.