This book is built on one sentence:

the one sentence which most of us can remember from history lessons at school, the one that reads "the assassination of Archduke Franz Ferdinand in Austria sparked the First World War", which Baldrick in Blackadder famously remembered as "a bloke called Archie Duke shot an ostrich". That sentence is supposed to be one of the settled facts of the historical narrative of the 20th century - so settled that few of us bother to search for the details, such as: who shot the duke? And why? How did he get there? And what happened to him afterwards?

Those questions are the starting point for Tim Butcher's elegant, horrifying and enlightening book - a book which is not only a good piece of detective work, it is the finest contribution so far this year to the rapidly expanding literature on the Great War. At the start, Butcher admits he also used to take the spark of the First World War for granted until, while covering the Bosnian war as a journalist in 1994, he found a tomb in Sarajevo that was being used as a latrine by the Bosnians who were under siege. When he discovered that the tomb belonged to Gavrilo Princip, a Bosnian who shot the Duke in the name of Slavic liberation, he began a long process of trying to understand why a man who fought for the freedom of his people should end up being despised by them.

The reason lies in the complex nationalism, identities and histories of Bosnia, Croatia and Serbia which Butcher untangles and explains by following the route that Princip took from this birthplace in Bosnia, through the schools where he was radicalised, to Sarajevo where he committed his revolutionary act. Where Princip went 100 years ago, Butcher goes again, which leaves this work feeling like a history book that has merged with a travel book and has the finest features of both. To that extent, it repeats the trick Butcher pulled off in Blood River, his 2004 account of his journey down the Congo.

The most memorable section, and the most beautifully written, is when he crosses the mountains to Bugojno, where Princip caught a train to Sarajevo. The landscape is, says Butcher, "a boundless, exposed dragon's back of a plateau without a single tree, its pelt of long grass combed backward and forwards by the shifting wind."

The chapter on the assassination itself is also particularly striking, especially as it is accompanied by a picture of the duke's car at the precise moment it turned into the street where the shooting took place. The car was supposed to go straight on, but no one had told the driver, so there it is in the picture: the car with its front wheels starting to turn. On the right, a man waves; on the left a little girl in a sun hat looks over her shoulder; and just beyond the camera's eye somewhere is Princip with a gun in his hand.

Butcher can't resist exploring the potential pre-destiny of that moment - he points out that the number plate of the car was AIIIII8 and that the war ended on the 11th day of the 11th month of 1918 or II-II-I8 - but the skill of the book lies not in any fantasies of synchronicity, but in his exploration of the consequences of the assassination for Princip and the world, and the possible lessons on nationalism and war. It is, in Butcher's words, a killing with more than one shadow.

One of those shadows is radicalisation and the story of how Princip lost interest in formal education, became interested in politics and in the end died for his cause: the expulsion from his homeland of what he saw as a foreign invader. The modern parallels present themselves, fully formed; they are one of Butcher's long shadows, one that stretches to the Bosnian War which was a training ground for the 9/11 jihadists.

As for nationalism, in exploring the history of Bosnia and the personal history of Princip, there are warnings in the book about ethnic division of all kinds. Princip unleashed forces, says Butcher, that in the end were hard to control and even turned against the man himself because he wasn't the right kind of Slav - it is a story of how, in his words, communities define themselves not by the similarity to the next community but by their difference. And Butcher's conclusion, reached by trekking through the mountains, is that the consequences of that are almost always negative.