When Archduke Franz Ferdinand was assassinated in 1914 few could have predicted it would lead to the battle of the Somme, whose first day would be the bloodiest in British military history.
The battle of the Somme began on July 1, 1916. On the first day almost 20,000 people were killed - four months later, more than a million soldiers had been killed or wounded on both sides of the fighting.
On the 100-year anniversary of the battle, the Queen and senior royals will lead remembrance services to honour the battle's victims. But what was the battle all about? We answer the key questions.
Why was the Somme offensive launched in the first place?
The Allies' original plan was for an attack on three European fronts in a bid to end years of trench warfare.
French and British commanders began discussions in late 1915 for a joint attack along a 25-mile front straddling the River Somme in Picardy in northern France.
But the German attack on French forces at Verdun in February 1916 meant the Allies had to alter their plan and bring more British soldiers into the attack.
Nigel Steel, historian at the Imperial War Museum, said: "The fighting (at Verdun) quickly drew in more and more French troops and it gradually became clear that the French army would be unable to sustain the defence of Verdun and lead an attack on the Somme. The proposed frontage of France's attack on the Somme was reduced from 25 to eight miles and, for the first time in the war, the main burden of the assault would be borne by the British."
What were the military objectives?
General Sir Douglas Haig, the British commander, aimed for troops to punch a huge hole in German lines. This would allow cavalry units to pour through and go east to Bapaume and north towards Arras, breaking the trench deadlock.
He did this by ordering a week-long artillery bombardment of more than a million shells.
Mr Steel added: "Haig's conception was one of breakout and return to open warfare. Yet his vision differed greatly from that of the man he selected to carry it out."
General Henry Rawlinson, whose troops were set to launch the attack, thought Haig's plan was overambitious. He favoured the 'bite and hold' technique in which modest sections of line were taken out and then defended against counter-attacks. But he was over-ruled, and told to prepare for a larger battle.
What happened when the attack started on July 1, 1916?
The bloodiest day in the history of the British Army. The week-long artillery barrage had failed to do anywhere near as much damage to the well-made German defences as had been hoped.
Many of the attacking forces were "pals battalions", volunteer units of friends and co-workers created the previous year in Lord Kitchener's New Army.
Many were told to walk slowly across no-man's land in a line because of their inexperience. They were slaughtered by machine gun fire.The attacking side lost 19,240 men killed, with 57,470 casualties overall.
Steel said: "It is hard, when discussing the battle as a whole, not to be brought up short by this enormously traumatic event and to award it a disproportionate amount of attention. It was there, on that day in particular, that the British attitude to war changed. The scale of the losses destroyed any patriotic enthusiasm that still lingered from 1914. There was now clearly going to be no triumphal march on Berlin. Instead, the war would be a long, slow grind paid out at great cost."
What about the rest of the Battle?
The battle itself dragged on for four months, with an average British casualty rate of 3,000 a day as it sank into 141 days of attrition.
The Battle of the Somme officially ended on November 18. The net gain was a strip of land 20 miles wide and six miles deep.
So was it a total failure?
Not necessarily. Even on the first day, the New Army units in the south of the Somme showed that they could fight, taking objectives from Fricourt to Montauban.
Also, a German officer famously described the battle as the "muddy grave of the German field army" and they eventually left the trenches of the Somme in early 1917.
Steel said: "The pressure of the Somme, exercised through attack and counter-attack, was keenly felt by the Germans."
Under new command from the end of August, the Germans began to plan for a withdrawal to new lines of defence some distance back.
He added: "When the Germans pulled back to these new positions in February and March 1917, the ground occupied by the BEF (British Expeditionary Force) and its French allies to the south represented as much a gain of the Battle of the Somme as that occupied over the 141 days of the battle itself."
What did the ordinary soldiers think?
"You get a lot of different viewpoints. But I think from the evidence of the original letters and diaries written at the time, the overwhelming thing you get is that the soldiers, whether they were soldiers or ordinary men, they felt very positive that they were actually going to make a big difference and this was going to be it," said Anthony Richards, the IWM's head of documents and sound and author of The Somme: A Visual History.
"Obviously, you get the odd one that will say 'I'm going to die, I'm not ready for this, I don't want to do it' but overwhelmingly there is this positive feeling that we have got to do this and we are going to make a difference.
"By the end you get a lot more cynicism about quite what they are doing and the battle kind of loses direction and it gets bogged down, so by November it has become a war of attrition, basically."