Royal intermarriage is the practice of members of ruling dynasties marrying into other reigning families. It was more commonly done in the past as part of strategic diplomacy for reasons of state. Although sometimes enforced by legal requirement on persons of royal birth, more often it has been a matter of political policy and/or tradition in monarchies.
In Europe, the practice was most prevalent from the medieval era until the outbreak of World War I, but evidence of intermarriage between royal dynasties in other parts of the world can be found as far back as the Late Bronze Age. Monarchs were often in pursuit of national and international aggrandisement on behalf of themselves and their dynasties, thus bonds of kinship tended to promote or restrain aggression. Marriage between dynasties could serve to initiate, reinforce or guarantee peace between nations. Alternatively, kinship by marriage could secure an alliance between two dynasties which sought to reduce the sense of threat from or to initiate aggression against the realm of a third dynasty. It could also enhance the prospect of territorial acquisition for a dynasty by procuring legal claim to a foreign throne, or portions of its realm (e.g., colonies), through inheritance from an heiress whenever a monarch failed to leave an undisputed male heir.
In parts of Europe, royalty continued to regularly marry into the families of their greatest vassals as late as the 16th century, thenceforth, tending to marry internationally. In other parts of the world royal intermarriage was less prevalent and the number of instances waxed and waned over time, depending on the culture and foreign policy of the time.
- 1 Royal marriage as international policy
- 1.1 Europe
- 1.2 Asia
- 1.3 Africa
- 1.4 Muslim World
- 2 Morganatic marriage
- 3 Inbreeding
- 4 Notes
- 5 References and Sources
Royal marriage as international policy
While the contemporary Western ideal sees marriage as a unique bond between two people who are in love, families in which heredity is central to power or inheritance (such as royal families) have often seen marriage in a different light. There are often political or other non-romantic functions that must be served, and the relative wealth and power of the potential spouses should be considered. Marriage for political, economic, or diplomatic reasons was a pattern seen for centuries among European rulers.
Medieval and Early Modern Europe
Careful selection of a spouse was important to maintain the royal status of a family: depending on the law of the land in question, if a prince or king was to marry a commoner who had no royal blood, even if the first-born was acknowledged as a son of a sovereign, he might not be able to claim any of the royal status of his father.
Traditionally, many factors were important in arranging royal marriages. One such factor was the amount of territory that the other royal family governed or controlled. Another, related factor was the stability of the control exerted over that territory: when there was territorial instability in a royal family, other royalty would be less inclined to marry into that family. Another factor was political alliance: marriage was an important way to bind together royal families and their countries during peace and war and could justify many important political decisions.
The increase in royal intermarriage often meant that lands passed into the hands of foreign houses, when the nearest heir was the son of a native dynast and a foreign royal.[n 1][n 2] The Habsburgs, for example, expanded their influence through arranged marriages and by gaining political privileges in what would become Switzerland, and in the 13th century the house aimed its marriage policy at families in Upper Alsace and Swabia. Given the success of the Habsburgs' territorial acquisition-via-inheritance policy, a motto came to be associated with their dynasty: Bella gerant alii, tu, felix Austria, nube! ("Let others wage war. You, happy Austria, marry!")
Monarchs sometimes went to great lengths to prevent this. On her marriage to Louis XIV of France, Maria Theresa, daughter of Philip IV of Spain, was forced to renounce her claim to the Spanish throne. When monarchs or heirs apparent wed other monarchs or heirs special agreements, sometimes in the form of treaties, were negotiated to determine inheritance rights. The marriage contract of Philip II of Spain and Mary I of England, for example, stipulated that the maternal possessions, as well as Burgundy and the Low Countries, were to pass to any future children of the couple, whereas the remaining paternal possessions (including Spain, Naples, Sicily, Milan) would first of all go to Philip's son Don Carlos, from his previous marriage to Maria Manuela of Portugal. If Carlos were to die without any descendants, only then would they pass to the children of his second marriage. On the other hand, the Franco-Scottish treaty that arranged the 1558 marriage of Mary, Queen of Scots and Francis, the son and heir of Henry II of France, had it that if the queen died without descendants, Scotland would fall to the throne of France.
Religion has always been closely tied to European political affairs, and as such it played an important role during marriage negotiations. The 1572 wedding in Paris of the French princess Margaret of Valois to the leader of France's Huguenots, Henry III of Navarre, was ostensibly arranged to effect a rapprochement between the nation's Catholics and Protestants, but proved a ruse for the St. Bartholomew's Day massacre. After the English Reformation, matches between English monarchs and Roman Catholic princesses were often unpopular, especially so when the prospective queen consort was unwilling to convert, or at least practice her faith discreetly.[n 3] Passage of the Act of Settlement 1701 disinherited any heir to the throne who married a Catholic. Other ruling houses, such as the Romanovs[n 4] and Habsburgs, have at times also insisted on dynastic marriages only being contracted with people of a certain faith or those willing to convert. When in 1926 Astrid of Sweden married Leopold III of Belgium, it was agreed that her children would be raised as Catholics but she was not required to give up Lutheranism, although she chose to convert in 1930. Some potential matches were abandoned due to irreconcilable religious differences. For example, plans for the marriage of the Catholic Władysław IV Vasa and the Lutheran Elisabeth of Bohemia, Princess Palatine proved unpopular with Poland's largely Catholic nobility and were quietly dropped.
Marriages among ruling dynasties and their subjects have at times been common, with the marriages such as that of Edward the Confessor, King of England with Edith of Wessex and Władysław II Jagiełło, King of Poland with Elizabeth Granowska being far from unheard of in medieval Europe. However, as dynasties approached absolutism and sought to preserve loyalty among competing members of the nobility, most eventually distanced themselves from kinship ties to local nobles by marrying abroad. Marriages with subjects brought the king back down to the level of those he ruled, often stimulating the ambition of his consort's family and evoking jealousy—or disdain—from the nobility. The notion that monarchs should marry into the dynasties of other monarchs to end or prevent war was, at first, a policy driven by pragmatism. During the era of absolutism, this practice contributed to the notion that it was socially, as well as politically, disadvantageous for members of ruling families to intermarry with their subjects and pass over the opportunity for marriage into a foreign dynasty.
While Roman emperors almost always married wives who were also Roman citizens, the ruling families of the empire's client kingdoms in the Near East and North Africa often contracted marriages with other royal houses to consolidate their position. These marriages were often contracted with the approval, or even at the behest, of the Roman emperors themselves. Rome thought that such marriages promoted stability among their client states and prevented petty local wars that would disturb the Pax Romana. Glaphyra of Cappadocia was known to have contracted three such royal intermarriages: with Juba II&I, King of Numidia and Mauretania, Alexander of Judea and Herod Archelaus, Ethnarch of Samaria.
Other examples from the Ancient Roman era include:
- Polemon II, King of Pontus and Berenice of Judea
- Aristobulus IV of Judea and Berenice of Judea
- Aristobulus Minor of Judea and Iotapa of Emesa
Though some emperors, such as Justin I and Justinian I, took low-born wives,[n 5] dynastic intermarriages in imperial families were not unusual in the Byzantine Empire. Following the fall of Constantinople in 1204, the ruling families, the Laskarides and then the Palaiologoi, thought it prudent to marry into foreign dynasties. One early example is the marriage of John Doukas Vatatzes with Constance, the daughter of Emperor Frederick II of the Holy Roman Empire to seal their alliance. After establishing an alliance with the Mongols in 1263, Michael VIII Palaiologos married two of his daughters to Mongol khans to cement their agreement: his daughter Euphrosyne Palaiologina was married to Nogai Khan of the Golden Horde, and his daughter Maria Palaiologina, was married to Abaqa Khan of the Ilkhanate. Later in the century, Andronikos II Palaiologos agreed martial alliances with Ghazan of the Ilkhanate and Toqta and Uzbeg of the Golden Horde, which were quickly followed by weddings with his daughters.
The Grand Komnenoi of the Empire of Trebizond were famed for marrying their daughters to their neighbours as acts of diplomacy.[n 6] Theodora Megale Komnene, daughter of John IV, was married to Uzun Hassan, lord of the Aq Qoyunlu, to seal an alliance between the Empire and the so-called White Sheep. Although the alliance failed to save Trebizond from its eventual defeat, and despite being a devout Christian in a Muslim state, Theodora did manage to exercise a pervasive influence both in the domestic and foreign actions of her husband.
Though usually made to strengthen the position of the empire, there are examples of interdynastic marriages destabilising the emperor's authority. When Emperor Andronikos II Palaiologos married his second wife, Eirene of Montferrat, in 1284 she caused a division in the Empire over her demand that her own sons share in imperial territory with, Michael, his son from his first marriage. She resorted to leaving Constantinople, the capital of the Byzantine Empire, and setting up her own court in the second city of the Empire, Thessalonica.
Post World War I era
In modern times, among European royalty at least, marriages between royal dynasties have become much rarer than they once were. Members of Europe's dynasties increasingly married members of the titled nobility, including Albert II of Belgium, George VI of the United Kingdom or Charles, Prince of Wales and very often commoners as Carl XVI Gustaf of Sweden, Harald V of Norway and Albert II, Prince of Monaco have done. In Europe, only Juan Carlos I of Spain, Elizabeth II of the United Kingdom and Alois, Hereditary Prince of Liechtenstein married members of a foreign dynasty.[n 7]
This being said, royal intermarriage still occurs. For example:
Members of two reigning houses
- Prince Nikolaus of Liechtenstein and Princess Margaretha of Luxembourg (1982, most recent example of intermarriage between two European dynasties reigning at the time of the wedding)
- Constantine II of Greece and Princess Anne Marie of Denmark (1964)
- Juan Carlos I of Spain and Princess Sophia of Greece (1962)
- Jean, Grand Duke of Luxembourg and Princess Joséphine Charlotte of Belgium (1953)
- Elizabeth II of the United Kingdom and Philip Mountbatten (of Greece and Denmark) (1947)
- Peter II of Yugoslavia and Alexandra of Greece and Denmark (1944)
- Frederick IX of Denmark and Ingrid of Sweden (1935)
- Prince George, Duke of Kent and Princess Marina of Greece and Denmark (1934)
- Umberto II of Italy and Marie José of Belgium (1930)
- Boris III of Bulgaria and Giovanna of Italy (1930)
- Olav V of Norway and Princess Märtha of Sweden (1929)
- Leopold III of Belgium and Astrid of Sweden (1926)
- Gustaf VI Adolf of Sweden and Lady Louise Mountbatten (1923)
- Alexander I of Yugoslavia and Maria of Romania (1922)
Members of one reigning house and one non-reigning house
- Princess Caroline of Monaco and Ernst August, Prince of Hanover (1999)
- Princess Doña María of Bourbon and Archduke Simeon of Austria (1996)
- Alois, Hereditary Prince of Liechtenstein and Duchess Sophie in Bavaria (1993)
- Princess Astrid of Belgium and Archduke Lorenz of Austria-Este (1984)
- Princess Marie-Astrid of Luxembourg and Archduke Carl Christian of Austria (1982)
- Princess Barbara of Liechtenstein and Prince Alexander of Yugoslavia (1973) 
- Princess Benedikte of Denmark and Richard, 6th Prince of Sayn-Wittgenstein-Berleburg (1968) 
- Princess Irene of the Netherlands and Carlos Hugo, Duke of Parma (1964)
- Princess Birgitta of Sweden and Prince Johann Georg of Hohenzollern (1961)
- Princess Alix of Luxembourg and Antoine, 13th Prince de Ligne (1950) 
Modern examples of dynastic intra-marriage
The Chakri Dynasty of Thailand has included marriages between royal relatives, but marriages between dynasts and foreigners, including foreign royals, are rare. This is in part due to Section 11 of 1924 Palace Law of Succession which excludes members of the royal family from the line of succession if they marry a non-Thai national.
The current king, Bhumibol Adulyadej is a first-cousin once removed of his wife, Sirikit, the two being, respectively, a grandson and a great-granddaughter of Chulalongkorn. Chulalongkorn married a number of his half-sisters, including Savang Vadhana and Sunandha Kumariratana; all shared the same father, Mongkut.
Marriage policy in imperial China differed from dynasty to dynasty. Tang dynasty (618–907) emperors gave their daughters in marriage to rulers of the Uyghur Khaganate to consolidate the special trade and military relationship that developed after the Khaganate supported the Chinese during the An Lushan Rebellion. At least three Tang imperial princesses are known to have married khagans between 758 and 821. These unions temporarily stopped in 788, which is believed in part to be because stability within the Chinese empire meant that they were politically unnecessary; however, threats from Tibet in the west, and a renewed need for Uyghur support, precipitated the marriage of Princess Taihe to Bilge Khagan.
Emperors of the proceeding Song dynasty (960–1279) tended to marry from within their own borders. Tang emperors, mainly took their wives from high-ranking bureaucratic families, but the Song dynasty did not consider rank important when it came to selecting their consorts. It has been estimated that only a quarter of Song consorts were from such families, with the rest being from lower status backgrounds. For example, Liu, consort of Emperor Zhenzong, had been a street performer and consort Miao, wife of Emperor Renzong was the daughter of his own wet nurse.
During the Qing dynasty (1644–1912), emperors chose their consorts primarily from one of the eight Banner families, administrative divisions that divide all native Manchu families. To maintain the ethnic purity of the ruling dynasty, after the Kangxi Period (1662–1722), emperors and princes were forbidden to marry non-Manchu wives. Imperial daughters however were not covered by this ban, however, and as with their preceding dynasties, were often married to Mongol princes to gain political or military support, especially in the early years of the Qing dynasty; three of the nine daughters of Emperor Nurhaci and twelve of Emperor Hongtaiji's daughters were married to Mongol Princes.
Japan and Korea
The Silla Kingdom had a practice that limited the succession to the throne to members of the seonggol, or "sacred bone", rank. To maintain their "sacred bone" rank, members of this caste often intermarried with one another in the same fashion that European royals intermarried to maintain a "pure" royal pedigree
The Japanese may not have seen intermarriage between them and the royal dynasties of the Korean Empire damaging to their prestige either. According to the Shoku Nihongi, an imperially commissioned record of Japanese history completed in 797, Emperor Kanmu who ruled from 781 to 806 was the son of a Korean concubine, Takano no Niigasa, who was descended from King Muryeong of Baekje, one of the Three Kingdoms of Korea.
More recently, In 1920, Crown Prince Yi Un of Korea was married to Princess Masako of Nashimoto and, in May 1931, Yi Geon, grandson of Gojong of Korea married Matsudaira Yosiko, a cousin of Princess Masako. The Japanese saw these marriages as a way to secure their colonial rule of Korea and introduce Japanese blood in to the Korean royal House of Yi.
At times, marriage between members of the same dynasty has been common in Central Africa. Marriages between the Swazi, Zulu and Thembu royal houses of southern Africa are common. For example, the daughter of former South African president and Thembu royal Nelson Mandela, Zeni Mandela, in 1977 married Prince Thumbumuzi Dlamini, a brother of Mswati III, King of Swaziland.
Examples of historical, mythical and contemporary royal intermarriages throughout Africa include:
- Mantfombi Dlamini, sister of Mswati III of Swaziland, and Goodwill Zwelithini, King of the Zulus, as his Great Royal Wife (1977)
- The Toucouleur emperor Umar Tall and the daughter of the sultan Muhammed Bello of Sokoto
From the time of the Umayyad conquest of Hispania and throughout the Reconquista, marriage between Spanish and Umayyad royals was not uncommon. Early marriages, such as that of Abd al-Aziz ibn Musa and Egilona at the turn of the 8th century, was thought to help establish the legitimacy of Muslim rule on the Iberian Peninsula. Later instances of intermarriage were often made to seal trade treaties between Christian kings and Muslim caliphs. Through these marriages, such as that of Alfonso VI of León and Castile and Zaida of Seville, it is believed that most European royalty can trace their ancestry back to the family of the prophet Muhammad, though this remains controversial.
The marriages of Ottoman sultans and their sons in the fourteenth and fifteenth centuries tended to be with members of the ruling dynasties of neighbouring powers. With little regard for religion, the sultans contracted marriages with both Christians and Muslims; the purpose of these royal intermarriages were purely tactical. The Christian Byzantines and Serbians, as well as the Muslim beyliks of Germiyan, Saruhan, Karaman and Dulkadir were all potential enemies and marriage was seen as a way of securing alliances with them. Marriage with foreign dynasties seems to have ceased in 1504, with the last marriage of a sultan to a foreign princess being that of Murad II and Mara Branković, daughter of the Serbian ruler Đurađ Branković, in 1435. By this time, the Ottomans had consolidated their power in the area and absorbed or subjugated many of their former rivals, and so marriage alliances were no longer seen as important to their foreign policy.
The Islamic principle of kafa'a discourages the marriages of women to men of differing religion or of inferior status.[n 9] Neighbouring Muslim powers did not start to give their daughters in marriage to Ottoman princes until the fifteenth century, when they were seen to have grown in importance. This same principle meant that, while Ottoman men were free to marry Christian women, Muslim princesses were prevented from marrying Christian princes.
Post World War I era
There are several modern instances of intermarriage between members of the royal families and former royal families of Islamic states (i.e., Jordan, Morocco, Saudi Arabia, the constituent states of the United Arab Emirates, etc.).
- Mohammed Reza Pahlavi of Iran and Princess Fawzia Fuad of Egypt (1939–1948)
- Sheikh Mohammed bin Rashid Al Maktoum of Dubai and Princess Haya bint Hussein of Jordan (2004)
- Muhammad Ali, Prince of the Sa'id, son of Fuad II of Egypt and Princess Noal Zaher Shah, granddaughter Zahir Shah of Afghanistan (2013)
- Ibrahim Ismail, Sultan of Johor and Raja Zarith Sofia of Perak (1982)
- Princess Durru Shehvar, daughter of Ottoman Emperor Abdülmecid II, and Azam Jah, son of Nizam of Hyderabad Asaf Jah VII (1931) 
At one time, some dynasties adhered strictly to the concept of royal intermarriage. The Bernadottes, Habsburgs, Sicilian and Spanish Bourbons and Romanovs, among others, introduced house laws which governed dynastic marriages; it was considered important that dynasts marry social equals (i.e., other royalty), thereby ruling out even the highest-born non-royal nobles. Those dynasts who contracted undesirable marriages often did so morganatically. Generally, this is a marriage between a man of high birth and a woman of lesser status (such as a daughter of a low-ranked noble family or a commoner). Usually, neither the bride nor any children of the marriage has a claim on the bridegroom's succession rights, titles, precedence, or entailed property. The children are considered legitimate for all other purposes and the prohibition against bigamy applies.
Examples of morganatic marriages include:
- Prince Alexander of Hesse and by Rhine and Countess Julia Hauke (1851)
- Duke Alexander of Württemberg and Countess Claudine Rhédey von Kis-Rhéde (1835)
- Grand Duke Constantine Pavlovich and Countess Joanna Grudna-Grudzińska (1796)
- Archduke Franz Ferdinand of Austria and Countess Sophie Chotek von Chotkova und Wognin (1900)
Over time, because of the relatively limited number of potential consorts, the gene pool of many ruling families grew progressively smaller, until all European royalty was related. This also resulted in many being descended from a certain person through many lines of descent, such as the numerous European royalty and nobility descended from Queen Victoria of the United Kingdom or King Christian IX of Denmark. The House of Habsburg was infamous for inbreeding, with the Habsburg lip cited as an ill effect, although no genetic evidence has proved the allegation. The closely related houses of Habsburg, Bourbon, Braganza and Wittelsbach[n 10] also engaged in first-cousin unions frequently and in double-cousin and uncle-niece marriages occasionally.
Examples of incestuous marriages and the impact of inbreeding on royal families include:
- All rulers of the Ptolemaic dynasty from Ptolemy II were married to their brothers and sisters, so as to keep the Ptolemaic blood "pure" and to strengthen the line of succession. Cleopatra VII (also called Cleopatra VI) and Ptolemy XIII, who married and became co-rulers of ancient Egypt following their father's death, are the most widely known example.
- Jean V of Armagnac was said to have formed a rare brother-sister liaison, left descendants and claimed to be married. There is no evidence that this "marriage" was contracted for dynastic rather than personal reasons.
- One of the most famous examples of a genetic trait aggravated by royal family intermarriage was the House of Habsburg, which inmarried particularly often and is known for the mandibular prognathism of the Habsburger (Unter) Lippe (otherwise known as the 'Habsburg jaw', 'Habsburg lip' or 'Austrian lip'"). This was typical for many Habsburg relatives over a period of six centuries.
- George I inherited the throne of Great Britain through his mother, Sophia of Hanover, a female line descendant of James VI and I.
- The crowns of the kingdoms of Aragon and Castile came under Habsburg rule when they were inherited by Charles V, Holy Roman Emperor, son of Joanna, Queen of Castile and Aragon and Philip the Handsome, son of Maximilian I, Holy Roman Emperor.
- A prime example is the marriage of the Catholic Henrietta Maria and Charles I of England. Her open practice of her faith and insistence on maintaining a Catholic retinue during a time of religious intolerance in English society eventually made her a deeply unpopular queen with the general public.
- Russian dynasts often only married foreign princesses when they converted to Russian Orthodoxy. For example, Alix of Hesse, wife of Nicholas II, converted from her native Lutheranism.
- Justin I's wife, Euphemia, was reported to be both a slave and a barbarian, and Justinian II's wife, Theodora, was an actor and, some claim, a prostitute.
- Donald MacGillivray Nicol says in The Last Centuries of Byzantium 1261–1453: "The daughters of Alexios II Grand Komnenos married the emirs of Sinope and of Erzindjan, his granddaughters married the emir of Chalybia and the Turkoman chieftain of the so-called Ak-Koyunlu, or horde of the White Sheep; his great-granddaughters, the children of Alexios III, who died in 1390, performed even greater service to the Empire."
- Both Elizabeth II of the United Kingdom and Juan Carlos I of Spain have married members of the Greek royal family, Prince Philip of Greece and Denmark and Princess Sophia of Greece and Denmark respectively. in 1993, Alois, Hereditary Prince of Liechtenstein married Duchess Sophie in Bavaria, a member of the House of Wittelsbach. Both the Greek royal dynasty, the House of Glücksburg, and House of Wittelsbach have been deposed.
- Knud, Hereditary Prince of Denmark and Princess Caroline-Mathilde of Denmark, married in 1933, were first cousins and members of the House of Glücksburg, as male-line grandchildren of Frederick VIII of Denmark.
- Carolyn Fluehr-Lobban explains in her article Islamic Law and Society in the Sudan that "It is preferable that a non-Muslim convert to Islam before marriage to a Muslim man, however, it is not essential - it is essential that a non-Muslim man convert to Islam before contemplating marriage with a Muslim woman"
- The Wittlesbach line suffered from several cases of mental illness, often attributed to their frequent intermarriages. Several family members suffered from mental and physical illnesses, as well as epilepsy
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