Nine reigning European kings, photographed together at Windsor in May 1910 for the funeral of Edward VII.

In May 1910, nine kings put on their dress uniforms and posed for a single photograph at Windsor Castle. They had gathered to bury one of their own — Edward VII of Britain — and the picture they left behind is one of the strangest documents of the twentieth century. Nearly every man in it was related to nearly every other. Within a decade, most of their thrones would be gone.

A map of Europe's royal houses in 1910 looks less like a political chart than a family tree draped over a continent. The Hohenzollerns held Germany. The Habsburgs held Austria-Hungary. The Romanovs held the largest land empire on earth. Britain's rulers belonged to the house of Saxe-Coburg and Gotha — a small German duchy that had, through a century of brilliant marriages, seeded its children into courts from Lisbon to Sofia. Sweden's crown belonged to the Bernadottes, descendants of one of Napoleon's marshals who had been offered a vacant throne in 1810 and simply kept it. His family reigns in Stockholm to this day.

The woman who tied the web together was Queen Victoria. Nine children, forty-two grandchildren, and a matchmaking policy pursued with the discipline of a foreign ministry earned her the nickname "the grandmother of Europe." By 1910 her descendants sat on, or beside, the thrones of Britain, Germany, Russia, Spain, Norway, Sweden, Romania and Greece. Kaiser Wilhelm II was her eldest grandchild. Tsarina Alexandra of Russia was her granddaughter. King George V was her grandson — and looked so much like his cousin Tsar Nicholas II that wedding guests once mixed them up.

The family knew each other by nicknames: Willy, Nicky, Georgie. They holidayed together, wore each other's uniforms as honorary colonels, and wrote letters full of family gossip. The assumption of the age — held by serious diplomats, not just sentimentalists — was that a Europe ruled by cousins could not tear itself apart.

There was a darker inheritance moving through the same bloodlines. Victoria carried the gene for hemophilia, and her matchmaking distributed it to the courts of Spain, Prussia and Russia. Its most fateful appearance was in Alexei, the only son of Nicholas and Alexandra. The desperate empress's reliance on the faith healer Grigori Rasputin — the one man who seemed able to stop the boy's bleeding — corroded the Russian monarchy's reputation from the inside during the very years it could least afford it. A single wedding gift of DNA helped destabilize an empire of 170 million people.

Nor were the marriages merely sentimental. For centuries, dynastic marriage had been Europe's cheapest foreign policy — a treaty signed in a chapel. What made the 1910 web unusual was that it had outlived its usefulness: by then, general staffs, alliance treaties and railway timetables governed war and peace, while the cousins exchanged birthday telegrams above it all. The form of family diplomacy survived; the substance had already leaked away. Almost no one noticed until it mattered.

July 1914 tested the theory directly. As Austria-Hungary moved against Serbia and the alliance system began to drag the great powers in, Willy and Nicky exchanged a series of personal telegrams — signed exactly that way — pleading with each other to hold back their armies. The "Willy–Nicky telegrams" are painful to read: two emperors, each insisting he wanted peace, each explaining that mobilization was out of his hands. The machinery their own governments had built proved stronger than the family that nominally commanded it.

The reckoning came fast. By the end of 1918 the Hohenzollerns had abdicated and the Habsburg empire had dissolved into successor states. The Romanovs fared worst: Nicholas, Alexandra and their five children were shot by Bolshevik guards in a basement in Yekaterinburg in July 1918. George V — who had worried about the optics of rescuing them — kept his throne, though his dynasty survived partly by rebranding: in 1917, with anti-German feeling running high, the British royal house quietly changed its name from Saxe-Coburg and Gotha to Windsor.

The 1910 map, then, is a portrait taken at the last possible moment: monarchical Europe at full strength, four years before the family business went bankrupt. Of the great dynasties on it, only the constitutional monarchies of the periphery — Britain, Scandinavia, the Low Countries, Spain — survive today.

Sources: Miranda Carter, George, Nicholas and Wilhelm: Three Royal Cousins and the Road to World War I; Margaret MacMillan, The War That Ended Peace; Encyclopaedia Britannica, "House of Windsor" and "Nicholas II"; the Willy–Nicky telegrams (July 1914, widely reproduced in WWI document collections).

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