PARIS - World War I was sparked by the assassination of a prince in Sarajevo 1914 and ended with an armistice signed in a French forest in 1918.
Here are 10 key moments in the conflict, which claimed the lives of some 10 million soldiers, brought down three empires and sowed the seeds of the communist revolution in Russia and World War II.
- Crown prince assassinated -
On 28 June 1914 the heir to the throne of the Austro-Hungarian empire, Archduke Franz Ferdinand, and his wife Sophie are visiting Sarajevo, the capital of Bosnia.
Bosnia is at the time a province of the empire, a situation resented by some Bosnians and neighbouring Serbia.
As the couple move through the city in a motorcade, nationalist Bosnian Serb student Gavrilo Princip opens fire and both are killed.
Austria-Hungary accuses Serbia of being responsible, setting off a chain of events that will in weeks lead the European powers into war, sucked in by a web of military alliances.
- Wars declared -
Austria declares war on Serbia on 28 July 1914 and attacks Belgrade.
Two days later Serbia's ally and protector, Russia, orders a general mobilisation of its army to intimidate Austria.
On 1 August Germany, ally of Austria, orders the same. France, allied with Russia, immediately follows.
Germany declares war on Russia the same day. On 3 August it declares war on France and its troops invade Belgium.
The next day, Britain -- allied with France and Russia -- declares war on Germany for violating the neutrality of Belgium.
- Battle of the Marne -
By early September 1914 German troops have advanced to just tens of kilometres outside Paris. The French government retreats southwest, to Bordeaux.
On 6 September French and British troops launch a desperate counter attack along the Marne River, northeast of the capital.
With the rail network around Paris in chaos, the military governor requisitions 700 taxis to bring 6,000 troops to the battlefront in the first military use of motorised transportation in history.
The battle is ferocious, a foretaste of what is to come, but German forces are eventually forced to pull back. Casualties are high: nearly 100,000 are dead or missing on both sides and double this number injured.
By 17 November the Western Front trench system stabilise, stretching from the North Sea to Switzerland, as both sides dig in for a war of attrition.
The conflict drags on in a bloody stalemate until the German offensives of March 1918.
- Gallipoli Campaign -
On 25 April 1915 British and French troops land on the Gallipoli peninsula in the Dardanelles Strait in Germany-allied Ottoman Turkey.
The Ottoman empire closed the straits at the start of the war, cutting off Russia from the Mediterranean.
The Allied campaign, pressed by Winston Churchill, aims to push through the straits and beyond, to attack Germany and Austria from the east and establish a link with Russia.
It is a catastrophe for the Allies, resulting in 180,000 deaths before they withdraw in January 1916.
Australia and New Zealand lose more than 10,000 men but Gallipoli becomes for them a defining event, marking the first time these newly independent countries fight under their own flags.
- Verdun, the longest battle -
On 25 February 1916 German forces launch an offensive at Verdun, east of Paris, to "bleed France white" and force the country to the negotiating table.
The German forces advance but are contained. By the time the fighting ends in December, the frontlines have barely changed despite staggering casualties.
More than 300,000 men are killed altogether, each side losing more or less the same number, and hundreds of thousands more are wounded.
Dragging on for more than 300 days, the Battle of Verdun is the longest of the war's Western Front.
- Somme, the bloodiest -
The 141-day Battle of the Somme is the bloodiest of the war, with more than a million casualties, including about 400,000 dead or missing.
It opens on 1 July 1916 as Allied forces -- mainly British -- aim to relieve pressure on the French, who are on the rack in Verdun, by attacking the Germans hundreds of kilometres to the northwest, around the Somme river.
The five-month battle is the deadliest in Britain's history, with 20,000 dead or missing in just the first hours, and 40,000 wounded.
When it is over, the Allies have advanced only a few kilometres.
- United States enters the war -
In January 1917 Germany -- under pressure from a British sea blockade -- steps up a campaign of attacking British merchant vessels with U-boat submarines, aiming to throttle the island.
It pushes the United States to enter the conflict, angered by the torpedoing of neutral ships in the Atlantic and vessels carrying US citizens.
Washington declares war on Germany on 6 April and on 26 June the first US deployment arrives at the French port of Saint-Nazaire.
By mid-1918 there are one million US troops in the conflict. By the war's end there will be two million and nearly 117,000 US soldiers will have died.
- French soldiers mutiny -
In mid-April 1917 the French army sends one million men into battle against the Germans at Chemin des Dames in Picardy.
The German resistance is strong and by the start of May the French have been able to advance only a few hundred metres at the cost of casualties that reach around 100,000 in just a few weeks.
French soldiers revolt, 30,000-40,000 taking part in various mutinies.
A month into the disastrous offensive, General Philippe Petain steps in and brings it to a halt.
The authorities are strict with the mutineers: 554 are sentenced to death and 49 executed.
The episode is remembered in Stanley Kubrick's "Paths of Glory" (1957), only released in France in 1975.
- Revolution in Russia -
Between 1914 and 1917 Russia loses more than two million soldiers and officers on the Eastern Front where its poorly equipped forces are decimated.
The conflict becomes highly unpopular.
Rioting snowballs into revolution in March 1917 and forces the abdication of Nicholas II, Russia's last tsar.
The new socialist-led government battles to impose control and does not envisage a withdrawal from the conflict.
There is a second revolution a few months later and Vladimir Lenin's Bolsheviks seize power. Their first decision is to seek a way out of the war.
By 15 December, Lenin has concluded an armistice with Germany. The Brest-Litovsk treaty that follows in March is a disaster for Russia, ceding huge chunks of its western territories to Germany -- including Poland, the Baltic states and Finland -- and slashing its population by nearly a third.
- Armistice -
Hit hard by four major German offensives in early 1918, the Allies -- reinforced by US troops -- launch a broad counter-offensive on 18 July, using tanks on a large scale to make the vital breakthrough which had eluded them for so long.
Planned by French forces commander General Ferdinand Foch, it changes the course of the war, forcing a German retreat on all sides.
Germany's allies begin to collapse: Bulgaria agrees an armistice by the end of September; Austria is defeated late October; and Turkey is forced to call a halt a few days later.
Germany is in turmoil and Kaiser Wilhelm II is forced to abdicate on 9 November.
On 11 November a German delegation, meeting Foch in a secluded forest setting north of Paris, agrees to an armistice.
Among the terms, Germany is to halt hostilities immediately and evacuate large areas it has occupied in less than a fortnight; it will surrender vast amounts of war materiel; all Allied prisoners of war are to be released.
The deal is signed at 5:20 am.
The ceasefire begins just hours later, at 11:00 am.
World War I is over.
The loss of life in WWI is visualised in this video.