On 28 June 1914, an event occurred that changed the course of history: the assassination of the heir-apparent to the throne of Austria-Hungary, Archduke Franz-Ferdinand, and his wife, Countess Sophie Chotek.

  Prof John Horne, TCD, on the assassination of Franz Ferdinand

Gavrilo Princip was the Bosnian Serb teenager who led the plot to assassinate the couple as they visited Bosnia in protest at the annexation of Bosnia by Austria-Hungary in 1908.

The first attempt to kill the couple failed as they drove to the City Hall.

But a wrong turning by the royal chauffeur on their return placed them by chance in front of Princip's revolver.

Austro-Hungarian outrage led exactly a month later, on 28 July 1914, to war with Serbia, which the government in Vienna blamed for the plot.

It Says in the Papers, 29 June 1914, with John S Doyle

Gallery: The Assassination of Franz Ferdinand

Blog: 100 years on, assassination still divides Bosnia

One week later, the so-called 'July Crisis' had turned into a European war, as Germany backed its Austro-Hungarian ally against Serbia, Russia upheld Serb independence with the support of France, and Britain and Ireland came in as Germany invaded France through neutral Belgium before turning (as it hoped) to deal with the Russians.

Assassinations of prominent figures were frequent in the early 20th century.

Why this one should have led in five weeks to a catastrophic war, and just who or what was responsible, were questions that dominated inter-war politics and helped cause World War II, as Germany rejected its supposed "war guilt".

It has remained one of the most complex issues in modern history because so many factors contributed to it without any one providing the key.

The men accused of the assassination of Archduke Franz Ferdinand and his wife are conducted into the court room: (1) Gavrilo Princip (2) Danilo Ilitch (3) N Cabrinovic

The role of Gavrilo Princip (who at 19 was too young to be executed and who died in an Austrian jail) is politically sensitive in Bosnia and Serbia to this day.

If we look briefly at the consequences of his action, however, we can suggest some of the pathways that led to the war for which Princip was definitely not responsible.

Firstly, historians still debate how much the Serb government knew of the plot, which was hatched in the Serb capital, Belgrade, and backed by radical Serb nationalists.

But by blaming Serbia and deciding to eliminate the country as an independent state, Austria-Hungary was defending its own existence as a multi-national, dynastic empire.

It had taken over Bosnia (from Ottoman Turkey) to prevent it becoming a spring-board for Serbia to cause trouble among the South Slav peoples ruled by Austria-Hungary.

In this sense, the crisis was a conflict between the dynastic principle and nationalism as the emergent political creed in Europe.

Secondly, Austria-Hungary knew that Russia, the self-declared protector of Serbia, was unlikely to allow its position to be weakened.

It could only act with Germany’s backing, since it could not take on Russia alone.

Here the Balkan crisis connected with the European balance of power, which was organised around two opposed alliances – the Triple Alliance of Germany, Austria-Hungary and Italy and the Triple Entente of Russia, France and the United Kingdom.

As the most powerful state on the continent, Germany had a pivotal role.

Coffins carrying the bodies of the assassination victims are carried through Sarajevo (Illustrated London News)

On 6 August it gave Austria-Hungary a "blank cheque" to proceed against Serbia, knowing this might lead to a European war.

Thirdly, Russia in turn felt that its own security and prestige rested on standing up to the Austro-German manoeuvre.

But this then involved the French, since France had allied with Russia in the early 1890s to overcome the isolation it had faced since defeat in the Franco-Prussian War of 1870.

The French and Russian governments did not seek war but saw it as a price they might have to pay to prevent Germany and Austria dominating south-eastern Europe.

Britain hesitated. It had little interest in the Austro-Serb quarrel but could not afford to see one power, Germany, dominate Europe.

When the German war plan led to the invasion of Belgium, however, the war became a "liberal crusade" for the preservation of small nations.

'The bomb thrower arrested' (Pic: Illustrated London News)

Ironically, for much of July 1914, few realised that Princip's act would lead to war.

The Irish Home Rule Crisis or the trial of the wife of a leading French politician, Madame Caillaux, who had shot a newspaper editor for publishing her intimate letters (she was exonerated), made better copy than yet another Balkan crisis that would presumably be resolved by the "concert" of the great powers.

It was the harsh Austrian ultimatum to Serbia on 23 July that precipitated war.

Yet this turned on the fourth key factor, which was what contemporaries understood by war.

The small group of decision-makers were conscious above all of playing a diplomatic game with very high stakes. The ace card was war.

But since for them war remained an instrument of politics, the risk of war was factored into the rules of the game and accepted by all the players.

It was Austria and Germany, by refusing any negotiation, which put the ace on the table. But Russia and France responded in kind.

Few understood that war would overturn both the game and the table, change their world beyond recognition and leave problems far greater than those that had occasioned it.

The fact that the City Hall in Sarajevo was destroyed by Bosnian Serb artillery during the siege in the 1990s is a poignant reminder that the events of a century ago are still read in the region through the violence that accompanied the break-up of Yugoslavia, the federal south Slav state that arose from the chaos of the Great War.

Sarajevo's Town Hall has now been restored

This ought to make Sarajevo a European "site of memory" to parallel that of Auschwitz.

For the city reminds us of how the local connects to the global, recalling as it does the descent into war and barbarity that marked 20th century Europe, and also of how the reconciliation that lies at the heart of the European project is the only possible answer.