[1] The current unified Kingdom of Denmark was founded or re-united by the Viking kings Gorm the Old and Harald Bluetooth in the 10th century.

The exact extent of Harald's kingdom is unknown, although it is reasonable to believe that it stretched from the defensive line of Dannevirke, including the Viking city of Hedeby, across Jutland, the Danish isles and into southern present day Sweden; Scania and perhaps Halland.

The reign of Cnut represented the peak of the Danish Viking age; his North Sea Empire included England (1016), Denmark (1018), Norway (1028) and held strong influence over the north-eastern coast of Germany.

Secondly, the Belts froze over in a rare occurrence during the winter of 1657–1658, allowing King Charles X Gustav of Sweden to lead his armies across the ice to invade Zealand.

In the following Treaty of Roskilde, Denmark–Norway capitulated and gave up all of Eastern Denmark (i.e. Skåne, Halland, Blekinge and Bornholm), in addition to the counties of Bohuslän and Trøndelag in Norway.

Three months after the peace treaty was signed, Charles X Gustav held a council of war where he decided to simply wipe Denmark from the map and unite all of Scandinavia under his rule.

Frederick III had stayed in his capital and now encouraged the citizens of Copenhagen to resist the Swedes, by saying he would "die in his nest", rather than to evacuate to safety in Norway.

Furthermore, this unprovoked declaration of war by Sweden finally triggered the alliance that Denmark–Norway had with the Netherlands, and a powerful Dutch fleet was sent to Copenhagen with vital supplies and reinforcements, which saved the city from being captured during the Swedish attack.

An official absolutist constitution, where absolute power and male primogeniture succession was laid down in the King's Law (Lex Regia) of 1665.

Frederick VII soon yielded to the Danish demands, and in March he accepted the end of absolutism, which resulted in the June Constitution of 1849.

[6] As King Frederick VII was without legitimate issue, Prince Christian of Glücksborg was chosen in 1853 as heir presumptive to the Danish throne, with the approval of the great powers of Europe, in light of the expected extinction of the senior line of the House of Oldenburg.

A justification for this choice was his marriage to Louise of Hesse-Kassel, who as a niece of Christian VIII, was a more close relative to the incumbent king than her husband.

Upon the death of King Frederick VII of Denmark in 1863, Christian IX acceded to the throne as the first Danish monarch of the House of Glücksburg.

Many Danish nationalists felt that Central Schleswig should be returned to Denmark regardless of the plebiscite's results, generally motivated by a desire to see Germany permanently weakened in the future.

Christian X agreed with these sentiments, and ordered Prime Minister Carl Theodor Zahle to include Central Schleswig in the re-unification process.

Subsequently, Christian X dismissed the rest of the government and replaced it with a de facto conservative care-taker cabinet under Otto Liebe.

This was the most recent time that a sitting Danish monarch made an executive decision without the support of a cabinet accountable to the legislature; following the crisis, Christian X accepted his drastically reduced role as symbolic head of state.

Following a referendum in 2009, the Act of Succession was amended so that primogeniture no longer puts males over females; a first-born child becomes heir to the throne regardless of gender.

[10] Today the monarch delegates much royal authority to Ministers in government, allowing the king to engage in the ceremonial role outlined by the Danish constitution.

The Prime Minister and Cabinet attend the regular meetings of the Council of State, at which the monarch presides and gives royal assent to laws.

The Danish Act of Succession[14] adopted on 27 March 1953 restricts the throne to those descended from King Christian X and his wife, Alexandrine of Mecklenburg-Schwerin, through approved marriages.

[15] Christian, Crown Prince of Denmark has been the heir apparent to the Danish throne since 14 January 2024, following the abdication of his grandmother, Margrethe II, and the ascension of his father, King Frederik X.

[16] The first law governing the succession to the Danish throne as a hereditary monarchy was the Kongeloven (Latin: Lex Regia), enacted 14 November 1665, and published in 1709.

As for the duchies, Holstein and Lauenburg where the King ruled as duke, these lands adhered to Salic law (meaning that only males could inherit the ducal throne), and by mutual agreement were permanently conjoined.

Originally, the Danish prime minister Christian Albrecht Bluhme wanted to keep the separate hereditary principles, but in the end the government decided on a uniform agnatic primogeniture, which was accepted by the Parliament.

This order of succession remained in effect for a hundred years, then the Salic law was changed to male-preference primogeniture in 1953, meaning that females could inherit, but only if they had no brothers.

Following the transformation of Denmark's monarchy from elective (at least theoretically, although it had generally descended to the eldest son of the House of Oldenburg since 1448) to hereditary in 1660, the so-called Kongelov (Latin: Lex Regia) established the right to rule "by the grace of God" for King Frederick III and his posterity.

Article 21 states "No Prince of the Blood, who resides here in the Realm and in Our territory, shall marry, or leave the Country, or take service under foreign Masters, unless he receives Permission from the King".

[17] Under this provision, princes of Denmark who permanently reside in other realms by express permission of the Danish Crown (i.e. members of the dynasties of Greece, Norway and the United Kingdom) do not thereby forfeit their royalty in Denmark, nor are they bound to obtain prior permission to travel abroad or to marry from its sovereign, although since 1950 those not descended in male-line from King Christian IX are no longer in the line of succession to the Danish throne.

Due to the morganatic status of her marriage, Marina, consort of Prince Michael, and their children, Princesses Alexandra and Olga, are exceptions.