The House of Habsburg was the most powerful dynasty of early modern Europe, ruling from Vienna to Madrid and, through marriage and inheritance, over lands on which the sun was said never to set. Its two great branches — the Spanish Habsburgs and the Austrian Habsburgs — descend from the shared inheritance of the Emperor Charles V, who in the sixteenth century held Spain, the Netherlands, much of Italy, the Holy Roman Empire, and the New World colonies, before dividing the burden between his son Philip II and his brother Ferdinand I.
No royal house makes the case for genogram analysis more starkly. The Habsburgs preferred to consolidate their empire by marrying within the family rather than dispersing it — uncle to niece, first cousin to first cousin, generation after generation. This deliberate policy of consanguineous marriage tightened their grip on territory but steadily narrowed the gene pool, and the family tree of the Spanish line in particular collapses inward, with the same ancestors appearing again and again where separate branches should be.
The consequences are written on the faces in the family's own portrait gallery. The pronounced protruding lower jaw and jutting lip known as the 'Habsburg jaw' — mandibular prognathism — grew more marked with each inbred generation, and modern genetic studies have directly linked its severity to the dynasty's rising coefficient of inbreeding. It reached its extreme in Charles II of Spain, so disabled by his inheritance that he could barely chew his food and left no children, ending the Spanish Habsburg line at his death in 1700 and igniting the War of the Spanish Succession.
The dynasty's own motto captured the strategy that both built and doomed it: 'Let others wage war; you, happy Austria, marry.' The Habsburgs conquered by wedding rather than fighting — and in the Spanish branch, that same instrument of empire became the mechanism of its extinction.
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