Christian IX of Denmark

Frederick VII's childlessness had presented a thorny dilemma and the question of succession to the Danish throne proved problematic. Denmark's adherence to the Salic Law and a burgeoning nationalism within the German-speaking parts of Schleswig-Holstein hindered all hopes of a peaceful solution. Proposed resolutions to keep the two Duchies together and part of Denmark proved unsatisfactory to both Danish and German interests. While Denmark had adopted the Salic Law, this only affected the descendants of Frederick III of Denmark, who was the first hereditary monarch of Denmark (before him, the kingdom was officially elective). Agnatic descent from Frederick III would end with the death of the childless King Frederick VII and his equally childless uncle, Prince Ferdinand. At that point, the law of succession promulgated by Frederick III provided for a Semi-Salic succession. There were, however, several ways to interpret to whom the crown could pass, since the provision was not entirely clear as to whether a claimant to the throne could be the closest female relative or not.

As the nations of Europe looked on, the numerous descendants of Helvig of Schauenburg began to vie for the Danish throne. Frederick VII belonged to the senior branch of Helvig's descendants. In 1863, Frederick, Duke of Schleswig-Holstein-Sonderburg-Augustenburg (1829–1880) (the future father-in-law of Kaiser Wilhelm II of Germany), proclaimed himself Frederick VIII, Duke of Schleswig-Holstein. Frederick of Augustenburg (as he was commonly known) had become the symbol of the nationalist German independence movement in Schleswig-Holstein after his father (in exchange for money) renounced his claims as heir to the throne of the duchies of Schleswig and Holstein. In view of the London protocol of 8 May 1852, which concluded the First War of Schleswig, and his father's concurrent renunciation to claims to the throne, Frederick's claim was not recognized by the parties to the protocol.

The closest female relatives of Frederick VII were his paternal aunt, Princess Louise Charlotte of Denmark, who had married a scion of the cadet branch of the House of Hesse, and her daughters. However, they were not agnatic descendants of the royal family, thus not eligible to succeed in Schleswig-Holstein.

The dynastic female heir reckoned most eligible according to the original law of primogeniture of Frederick III was Caroline of Denmark (1793–1881), the childless eldest daughter of the late king Frederick VI. Along with another childless daughter, Wilhelmine of Denmark (1808–1891), Duchess of Glücksburg; the next heir was Louise, sister of Frederick VI, who had married the Duke of Augustenburg. The chief heir to that line was the selfsame Frederick of Augustenburg, but his turn would have come only after the death of two childless princesses who were very much alive in 1863.

The House of Glücksburg also held a significant interest in the succession to the throne. A more junior branch of the royal clan, they were also descendants of Frederick III through the daughter of King Frederick V of Denmark. Lastly, there was yet a more junior agnatic branch that was eligible to succeed in Schleswig-Holstein. There was Christian himself and his three older brothers, the eldest of whom, Karl, was childless, but the others had produced children, and male children at that.

Prince Christian had been a foster "grandson" of the "grandchildless" royal couple Frederick VI and his Queen consort Marie (Marie Sophie Friederike of Hesse). Familiar with the royal court and the traditions of the recent monarchs, their young ward Prince Christian was great-nephew of Queen Marie and descendant of a first cousin of Frederick VI. He had been brought up as a Dane, having lived in Danish-speaking lands of the royal dynasty and not having become a German nationalist, which made him a relatively good candidate from the Danish point of view. As junior agnatic descendant, he was eligible to inherit Schleswig-Holstein, but was not the first in line. As a descendant of Frederick III, he was eligible to succeed in Denmark, although here too, he was not first-in-line.

In 1842, Christian married Princess Louise of Hesse-Kassel, daughter of the closest female relative of Frederick VII. Louise's mother and brother, and elder sister too, renounced their rights in favor of Louise and her husband. Prince Christian's wife was now the closest female heiress of Frederick VII.

In 1852, the thorny question of Denmark's succession was resolved by the London Protocol of 8 May 1852, through which Christian was chosen as next-in-line for the throne after Frederick VII and his uncle. The decision was implemented by the Danish Law of Succession of 31 July 1853—more precisely, the Royal Ordinance settling the Succession to the Crown on Prince Christian of Glücksburg[5]—which designated him as heir to the entire Danish monarchy following the extinction of the male line of Frederick III and granted him the title Prince of Denmark.

The six children of Christian IX and Queen Louise, photographed in 1882. From the left: King George I of Greece, Empress Maria Feodorovna of Russia, Alexandra, Princess of Wales, Crown Prince Frederick of Denmark, Princess Thyra and Prince Valdemar.

Christian's family links with Europe's royal families earned him the sobriquet "the father-in-law of Europe". Four of Christian's children sat on the thrones (either as monarchs or as consorts) of Denmark, Greece, the United Kingdom and Russia.

His daughter Thyra could have become Queen of Hanover had her husband, Prince Ernest Augustus, not been deprived of the throne of Hanover upon its annexation by Prussia in 1866. His youngest son, Valdemar, was offered the crown of Bulgaria, but had to decline under international pressure.

The great dynastic success of the six children was to a great extent not attributable to Christian himself, but the result of the ambitions of his wife Louise of Hesse-Kassel. Some have compared her dynastical capabilities to those of Queen Victoria of the United Kingdom. An additional factor was that Denmark was not one of the Great Powers, so the other powers did not fear that the balance of power in Europe would be upset by a marriage of one of its royalty to another royal house.

Christian's grandsons included Nicholas II of Russia, Constantine I of Greece, George V of the United Kingdom, Christian X of Denmark and Haakon VII of Norway.

Today, most of Europe's reigning and ex-reigning royal families are direct descendants of Christian IX, and most current European monarchs are descended from him, including Queen Margrethe II of Denmark, Queen Elizabeth II of the United Kingdom, King Philippe of Belgium, King Harald V of Norway, King Felipe VI of Spain and Grand Duke Henri of Luxembourg. The consort Prince Philip, Duke of Edinburgh, and former consort Queen Sofía of Spain are also agnatic descendants of Christian IX, as is Constantine II, the former and last King of the Hellenes, and his consort the former Queen Anne-Marie. King Michael I of Romania and his wife Queen Anne of Romania were also descendants of Christian IX.[7]

• Bramsen, Bo (1993). Huset Glücksborg. Europas svigerfader og hans efterslægt (in Danish) (2nd ed.). Copenhagen: Forum. ISBN 87-553-1843-6.
• Fabricius-Møller, Jes (2013). Dynastiet Glücksborg, en Danmarkshistorie (in Danish). Copenhagen: Gad. ISBN 9788712048411.
• Olden-Jørgensen, Sebastian (2003). Prinsessen og det hele kongerige. Christian IX og det glücksborgske kongehus (in Danish). Copenhagen: Gad. ISBN 8712040517.
• Scocozza, Benito (1997). Politikens bog om danske monarker (in Danish). Copenhagen: Politikens Forlag. ISBN 87-567-5772-7.