Early Modern States of Scandinavia and the Baltic: Map & History

The three kingdoms of Denmark, Norway and Sweden united in the Kalmar Union in 1397. Sweden broke out of this union and re-entered it several times, until 1521, when Sweden finally left the Union, leaving Denmark–Norway (including overseas possessions in the North Atlantic and the island of Saaremaa in modern Estonia). During the Count's Feud, where the Danish crown was contested between Protestant Oldenburg King Christian III and Catholic King Christian II, the relatively Catholic realm of Norway also wished to leave the union in the 1530s, but was unable to do so due to Denmark's superior military might. Following this, King Christian III completely united the two kingdoms to reduce the chances of further uprisings, and made Lutheranism the state religion in both kingdoms.

From Wikipedia: Denmark–Norway · History: Origins of the Union (CC BY-SA 4.0)

Christian III, who had relied on Swedish aid in the Count's Feud, kept peaceful relations with Sweden throughout his reign. However, Frederick II was quite hostile towards the Swedes.

From Wikipedia: Denmark–Norway · History: Northern Seven Years' War (CC BY-SA 4.0)

Protestantism had been a religious movement in Denmark ever since the reign of Christian II. Though the country remained Catholic during the reign of Frederick I, and in Norway it was not a big movement at that time. But the victory in the Count's Feud secured Denmark under the Protestant King Christian III, and in 1537 he also secured Norway, creating the union between the two kingdoms.

From Wikipedia: Denmark–Norway · Culture: Religion (CC BY-SA 4.0)

Denmark–Norway was defeated and King Frederick VI had to cede the Kingdom of Norway to the King Charles XIII of Sweden 14 January 1814 at the Treaty of Kiel, but Norway's overseas possessions were kept by Denmark. The Norwegians objected to the terms of this treaty, and a constitutional assembly declared Norwegian independence on 17 May 1814 and elected the Crown Prince Christian Frederik as king of independent Norway. Following a Swedish invasion, Norway was forced to accept a personal union between Sweden and Norway, but retained its liberal constitution and separate institutions, except for the foreign service. The union was dissolved in 1905.

From Wikipedia: Denmark–Norway · History: The end of the Union (CC BY-SA 4.0)

Although the Dano–Norwegian union was generally viewed favourably in Denmark and Norway at the time of its dissolution in 1814, some 19th-century Norwegian writers disparaged the union as a "400-year night". Some modern historians describe the idea of a "400-year night" as a myth that was created as a rhetorical device in the struggle against the Swedish–Norwegian union, inspired by 19th-century national-romanticist ideas.

From Wikipedia: Denmark–Norway · Legacy (CC BY-SA 4.0)

Sweden emerged as a great European power under Axel Oxenstierna and King Gustavus Adolphus. As a result of acquiring territories seized from Russia and the Polish–Lithuanian Commonwealth, as well as its involvement in the Thirty Years' War, Sweden found itself transformed into the leader of Protestantism.

From Wikipedia: Swedish Empire · History: Emergence as a great power (CC BY-SA 4.0)

A major reason why Sweden could be so successful in wars with such a scarce number of soldiers was its advanced military tactics. Sweden was able to reform its military tactics continuously throughout the period. Prior to Gustavus Adolphus's reforms, both his father, Charles IX, and his uncle Erik XIV had tried to reform the army but had effectively failed to do so. Charles IX, like most other rulers, had tried to implement the Dutch system into the army but with limited success. The lack of a strict organisation in the infantry caused the proportion of pikemen to musketeers to be far lower than the preferred ratio of 1 to 1. This, combined with the lack of funds to provide the soldiers with armour, caused the Swedish infantry to be dangerously lightly equipped and unable to deal with cavalry or heavier infantry in open terrain.

From Wikipedia: Swedish Empire · Military history (CC BY-SA 4.0)