London: At 7:00 pm on August 4, 1914, Sir Edward Goschen, Britain`s ambassador to Berlin, went to the foreign office on Wilhelmstrasse and issued London`s ultimatum: withdraw from Belgium by midnight, or face war.
The lack of response meant that five hours later Britain, and her empire, entered the war, transforming the burgeoning European conflict into a global one.
Prime minister Herbert Henry Asquith`s government had demanded Germany respect Belgium`s neutrality after troops crossed the border earlier in the day.
But instead of satisfaction, Goschen was given his passport.
"His Majesty`s Government has declared to the German Government that a state of war exists between Great Britain and Germany as from 11 pm on August 4th", said a Foreign Office statement.
The final moment came only after long reflection and heated debate in Britain.
The initial objective of the foreign secretary Edward Grey "had been to prevent the outbreak of war", said William Mulligan, a historian at the University of Dublin`s Centre of War Studies, in his book "The Great War for Peace".
However, on August 3, he told parliament: "It is clear that the peace of Europe cannot be preserved."The descent into war was triggered by the assassination by a Serbian nationalist of the archduke Franz Ferdinand, heir to the Austro-Hungarian throne, on June 28 in Sarajevo.
A complex web of European alliances then came into play, sparking a domino effect of decisions that sucked in the major powers of the day.
Austria-Hungary declared war on Belgrade on July 28, bombing the Serbian capital the following day.
A day later, Serbia`s ally Russia began mobilising. On August 1, Germany declared war on Russia and two days later on France. On August 4, with Germany invading neutral Belgium, Britain declared war on Germany.
"Britain`s entry to the war transformed the conflict into a global one," Mulligan told AFP.
"Fighting spread to Africa, Japan joined the war in the Far East, seizing some German colonies, and soldiers from the French and British empires bolstered their armies in Europe."
When Britons learned they were at war, they believed -- along with the peoples of Europe -- the conflict would be a short one.
"The shouts of newsboys were the principal means by which people found out about the declaration of war," said Catriona Pennel, senior lecturer in history at the University of Exeter.
"Owing to the late time of the expiration of the ultimatum and the obvious fact that there was no radio or television in 1914, most people in the UK discovered they were at war the following morning," she told AFP.
The Financial Times newspaper`s headline read: "European War: Britain to take part in this struggle."
"Great Britain declares war on Germany" anounced The Manchester Guardian, while The Times headlined: "War Declared: Note rejected by Germany".
In the cities, the news Britain was at war spread with the early morning papers. Word took longer to reach rural areas, often brought in person by police officers or postmen.In the hours building up to the August 4 declaration of war, a crowd had gathered in London`s Trafalgar Square in support of British intervention.
The rally prompted observers such as the philosopher and pacifist Bertrand Russell to conclude that "average men and women were delighted at the prospect of war".
Public rallies and the massive numbers who went on to volunteer to fight -- more than 750,000 in August and September 1914 alone -- has fuelled the notion of a groundswell of British support for the war.
But according to historians, popular enthusiasm was limited at the time.
"The coming of war was like a dark storm cloud over people who feared for the future of European civilisation," said Pennel.
In a country without conscription, signing up to fight relied more on patriotism and the will to defend Britain than on genuine enthusiasm for war.
"Many volunteers considered joining the army to be a duty," said Mulligan.
Volunteering was also motivated by "fear of a German victory, which seemed probable after the rapid advances through Belgium and northern France".
To counter the German advance, the British Expeditionary Force under the command of Sir John French was swiftly dispatched, landing in France on August 7. Though it helped to stop the German advance, the BEF suffered huge casualties.
And contrary to the early belief the troops would be home for Christmas, the war lasted until November 11, 1918, at a cost of 10 million military and millions more civilian lives.