the greatest error of modern history?

Prince Archduke Francis Ferdinand [1863-1914], heir to the Austro-Hungarian Empire, was assassinated in Sarajevo. The Archduke had been on a visit to Bosnia. The assassin was a Serbian nationalist whose co-conspirators were in Belgrade. The Serbians wanted to detach Bosnia from the Austro-Hungarian Empire.

A look back at a review of Niall Ferguson's [born 1964] World War I book The Pity of War where he argues that Britain's intervention in 1914 was "nothing less than the greatest error of modern history," because Germany in fact did not pose an essential threat to British interests.