The chaos of the Habsburg family tree brought down the dynasty and is one of the most famous cases of royal inbreeding throughout history. The Habsburg line traces its roots to the Middle Ages and extended its influence through the early 20th century. Some of the most famous names in European history had links to the family: Marie Antoinette was a Habusburg, as was Archduke Franz Ferdinand, the man whose death triggered the start of World War I.
Habsburg dynastic power reached its height in the 16th and 17th centuries. As the Spanish and Austrian lines of the House of Habsburg dominated Europe, they married one another and kept bloodlines pure. Cousins married cousins and uncles married nieces; keeping track of who was related to who in the complex matrix of the House of Habsburg inbreeding - not to mention how they were related - is mind-boggling to the modern observer.
Habsburg inbreeding and intermarriage brought together bloodlines in a way that caused physical and mental illnesses, ultimately resulting in infertility on a royal scale. Confusing and fascinating, the Habsburg dynasty inbred itself into oblivion.
The founding of the Habsburg Empire is attributed to Rudolf I, who became King of Germany in 1273. He was a member of the Habsburg family, which had been around since the 11th century. With his election as king (yes, he was elected king), he brought together extensive Germanic lands under a single leader. Territory under his control extended in 1276, when he seized Austria. In 1281, he gave his Austrian possessions to his son, Albert. From that point on, Austria and the House of Habsburg were linked. The Habsburgs added Bohemia and Hungary to their growing empire, and continued to acquire land and power for centuries, through both military action and diplomacy.
When Maximilian I, son of the Holy Roman Emperor Frederick III, married Mary, the daughter of French King Charles the Bold in 1477, the Habsburgs extended their European influence immensely. The marriage resulted in Maximilian, who later became Holy Roman Emperor, gaining control of the Netherlands, Luxembourg, and parts of France. He later married Bianca, daughter of the Duke of Milan, after Mary's death.
The passing of Mary caused various problems for Maximilian; he had to fight to maintain control over the Netherlands, which he gained in his marriage to her. He also struggled to maintain control of Hungary, though he ultimately did. However, by his death, in 1518, he had lost ground in Switzerland.
Maximilan's biggest contribution to the Habsburg dynasty may have been securing his son Philip's marriage to Joann of Castile. Joann, also known as Joan the Mad, was the daughter of Ferdinand and Isabella of Spain, and came with a lot of money, land, and prestige. The vast swaths of land and royal families infiltrated by the Habsburgs led directly to the culture of inbreeding that ultimately destroyed the family.
Maximilian's son Philip married Joann of Castile in 1496. As the daughter of Ferdinand and Isabella of Spain, Joann brought numerous possessions and great prestige to the Habsburg dynasty. When Joann inherited Castile, after her mother's death in 1504, her father acted as regent. By 1506, Philip was actively vying for control; he negotiated a treaty with Ferdinand to give Castile completely over to Joann. Due to claims of his wife's failing mental health, Philip took complete authority Castile, thereby formally linking the Spanish and Austrian houses of the Habsburg lines.
As for Joann's mental health, according to a piece in The Journal of Humanistic Psychiatry, the queen, aware many believed her mad, wrote a letter "denying insanity, [and] stating that she simply had jealousy issues that she believed she had probably inherited from her mother." It remains unclear whether she suffered from psychological ailments or was a political puppet. She was the product of a marriage between second cousins, which may have contributed to any psychological disorder she had.
Historians speculate Joann may have suffered from depression or bipolar disorder, but it's possible this was exaggerated by her husband and father for their own gains. Philip only lived for a few months after he declared Joann incompetent to hold the crown of Castile. After his death, Ferdinand re-took control, and had Joann confined to prison, which may have caused her mental health to fail. After Fredinand died in 1517, Joann's son Charles, later Holy Roman Emperor Charles V, inherited Castile and other Spanish possessions.
During the early 16th century, Habsburg marriages created a dynasty that touched most of Western Europe and, by extension, explored the New World. In addition to Charles I of Spain becoming Holy Roman Emperor Charles V, his sister Isabella married into the royal house of Denmark and his brother, Ferdinand, who later became Holy Roman Emperor as well, married Anne of Bohemia and Hungary.
Given the massive reach of the Habsburgs, there were concerns over maintaining power, and difficulty in arranging marriages to do so. Charles V's daughter, Maria of Spain, married her cousin Maximilian (son of Ferdinand and Anne) in 1548; his son, Philip, married Anna of Austria, daughter of Maria and Maximilian, who was his niece. Keeping the bloodlines connected was ideal for dynastic power, though such marriages kicked off increasingly close links in kinship.
That said, cousins marrying one another wasn't new or scandalous. In the 12th century, Eleanor of Aquitaine married her fourth cousin, Louis VII of France, before later marrying Henry II of England. Louis VII married his second cousin, Constance. Henry VIII married several relatives, and Isabella and Ferdinand of Spain were second cousins.