PARIS - On August 2, 1914 , a young French corporal called Jules-Andre Peugeot and a German lieutenant, Albert Mayer, died in a firefight in Joncherey, northeast France, the first official casualties of World War I.
Ten million were to follow on battlefields from the bitter North Sea coast to Russian steppes and the deserts of the Middle East.
The fate of those young soldiers, the first was 21, the second 22, was sealed in the week of July 28 to August 4, 1914, that marked the plunge into four horrific years of war.
Compared with World War II - where Nazi Germany's role as aggressor is uncontested - the root causes of the Great War of 1914-18 remain a matter for intense historical debate.
But what is well known is the clockwork-like sequence that set the war in motion.
Europe in 1914 was divided into two allied blocs - with the Triple Alliance of Germany, Italy and the Austro-Hungarian Empire on the one side. On the other were France, Russia and Britain, known as the Triple Entente.
As schoolchildren the world over are taught, the assassination of Archduke Franz Ferdinand - heir to the Austro-Hungarian throne - by a nationalist Serb student in Sarajevo on June 28, lit the fuse.
The Austro-Hungarians saw a chance to slap down their Serb neighbours, who they accused of backing pan-Slav nationalism within the empire.
A month later, on July 28, Austria-Hungary declared war on Serbia, sending in cannons to shell Belgrade, having received assurances of German support.
From that point onwards, each decision by one bloc led, as in a game of diplomatic chess, to a response by the other.
Russia was drawn into the conflict in aid of its Serbian ally, declaring a general mobilisation on July 30.
On August 1, Germany declared war on Russia and mobilised its own army. France also mobilised its troops in solidarity with its Russian ally.
Sweeping military plans
Squeezed between a hostile France in the west and Russia in the east, explains British historian Michael Howard, Germany's military planners had long decided that to gain a victory over Russia, its long-term rival France must first be defeated.
That was how Germany came to invade France, as the opening campaign of a conflict to which Paris was not originally party.
On August 3, Germany declared war on both France and Serbia, and sent its troops marching through Belgium towards the French border.
Britain declared war on Germany the following day, honouring the terms of a defence treaty with Belgium.
On both sides of the Franco-German border, two sweeping military strategies, the "Plan XVII" in France and the German "Schlieffen Plan" were set in motion.
In France, farmers rushed to finish harvesting their crops and young men flocked to Paris where soldiers packed into trains scrawled with the slogan "To Berlin!".
The national anthem, the Marseillaise, was heard at all hours of the day at the Gare de l'Est station, as soldiers bellowed out "We are going to cut off Wilhelm's moustache" - in defiance of the German emperor.
But things played out quite differently on the ground.
German artillery overwhelmed Belgian defences and von Moltke's troops drove deep into northern France, catching Joffre by surprise and pushing the advancing French lines back to the Marne river, within 45 kilometers (28 miles) of Paris.
Two major campaigns, the Battle of the Marne in September and the First Battle of Ypres in October, failed to deliver a conclusive outcome despite horrific casualties, with some 250,000 killed or wounded on either side.
Instead the German and Allied forces were left facing off from trenches hollowed out of the mud of northern and eastern France, in positions they would hold for much of the war and which became known as the Western Front.
Wary of their neighbours
Military leaders in France and Germany had roughly equivalent troop numbers at their disposal, and both had expected a short war.
But those hopes were soon dashed. By the autumn of 1914, 19 countries had been drawn into war.
Italy was to side with the Entente in 1915, in spite of earlier treaties aligning it with Germany. The Central Powers of Germany and Austria-Hungary, as they became known, were joined by Turkey and Bulgaria.
Was war inevitable? Europe was not expecting a war but the conditions were ripe for one, with a powderkeg mix of populist pressure, national pride and political miscalculations.
Across the continent, nationalist movements were in the minority in 1914, but its peoples were by and large strongly patriotic - and wary of their neighbours' intentions.
Nations were quick to perceive themselves as victims of aggression, or in Germany's case, attempts to keep it from its rightful place in the world.
When the war did come, lawmakers in many countries voted overwhelmingly in favour of credits to wage it - even in pacifist Britain, swayed both by concerns for the rights of small nations, and for the balance of power in Europe.