Habsburg Dynasty
The European dynasty that used marriage instead of war to assemble an empire from Madrid to Vienna, and ruled parts of it for six centuries.
A Latin tag followed this family for four hundred years: let others wage war, you, happy Austria, marry. It was unfair, since the Habsburgs fought constantly, but it caught something true. No dynasty in European history acquired more territory through the marriage contract, and none was undone more completely by the thing marriage could not create, which was a nation.
How a minor count became a superpower
The family began as counts in what is now Switzerland. In 1273 the German electors chose Rudolf of Habsburg as king of the Romans precisely because he seemed too weak to threaten them, a miscalculation with a long tail. Rudolf took Austria and never gave it back.
The great acquisitions came in three marriages. In 1477 Maximilian I married Mary of Burgundy and gained the Low Countries, the richest urban economy in Europe. Their son Philip married Joanna of Castile, heiress to a newly united Spain. Their son Charles, born in Ghent in 1500, inherited the lot: Spain and its American silver, the Netherlands, Austria, southern Italy. In 1519 he bought his election as Holy Roman Emperor with money borrowed from the Fugger bank. Charles V ruled more of the world than anyone since the Romans and spent his reign exhausted by it, fighting France, the Ottomans, and Martin Luther at once. In 1556 he abdicated and split the inheritance: Spain and the Netherlands to his son Philip II, Austria and the imperial title to his brother Ferdinand.
Two branches, two failures
The Spanish branch had the money and lost it. American silver funded a century of war against the Dutch, the English, and the French, and Spain declared bankruptcy repeatedly while the silver was still arriving. The line ended in 1700 with the childless and gravely disabled Charles II, whose ancestry included so much cousin marriage that his inbreeding coefficient exceeded that of a brother-sister union. His death triggered the War of the Spanish Succession, settled at the Peace of Utrecht in 1713.
The Austrian branch had the problem that outlived it. Its lands were a patchwork of Germans, Hungarians, Czechs, Poles, Ukrainians, Romanians, Croats, Slovenes, Serbs, and Italians held together by a dynasty rather than a shared identity. This was unremarkable in 1700 and lethal after 1789, when nationalism made the question of which nation a state belonged to the only question that mattered. Napoleon dissolved the Holy Roman Empire in 1806 and the Habsburgs simply renamed their holdings the Austrian Empire. The revolutions of 1848 nearly ended them. The 1867 compromise with Hungary bought fifty more years by promoting one subject nation to co-owner and leaving the Slavs out, which guaranteed that the Slavs would become the problem.
The end
Franz Joseph reigned for sixty-eight years, long enough that his subjects could not imagine the state without him. When his heir Franz Ferdinand was shot in Sarajevo in 1914, Vienna treated a Serbian conspiracy as an existential question about whether a multinational empire could survive a nationalist neighbor. It was right about the stakes and catastrophically wrong about the remedy. In November 1918 Karl I renounced participation in government, and the empire dissolved into the small states that still occupy central Europe.
What they leave behind
The Habsburg question was never answered, only postponed: how do you govern many nations in one state without a nation? The successor states tried ethnic homogeneity, which required expulsion and worse. The European Union is, among other things, a second attempt at the same problem with better manners.