Royalty, political leaders and relatives of the fallen united tonight to remember the sacrifices and losses exactly a century on from Britain's entry into the First World War.
A moving twilight ceremony at St Symphorien military cemetery near Mons was the highlight of a day of events in the UK and Belgium marking 100 years since the Great War's start.
Stark first-hand accounts of life on the front line were heard as darkness fell in the hushed woodland cemetery.
Lit by spotlights, the cemetery - the final resting place for men who once fought as enemies - provided an evocative backdrop to the tales of horror.
The Duke and Duchess of Cambridge, Prince Harry and Prime Minister David Cameron stood shoulder to shoulder with counterparts from countries including France and Germany.
Mr Cameron said the Great War, which claimed millions of lives, including 750,000 from the British and Commonwealth, was "unlike any other".
In front of 500 guests, including German president Joachim Gauck, Irish President Michael D Higgins and the Archbishop of Canterbury, Mr Cameron said: "Every war is cruel.
"But this war was unlike any other. The unspeakable carnage, the unbearable loss, the almost unbelievable bravery."
Prince Harry read a letter from Private Michael Lennon, of 1st Battalion the Royal Dublin Fusiliers, dated May 30 1915, who wrote to his brother Frank the day before he was due to land in Gallipoli.
He was killed in action on June 28 1915 - exactly a year after Archduke Franz Ferdinand was assassinated in Sarajevo, sparking the war.
In the letter, Pte Lennon wrote: "Well Frank, I suppose we are for it tomorrow, if we don't get shelled on the way."
A touching letter to the War Office from the mother of John Parr, believed to be the first British soldier to die on the Western Front and who is buried at St Symphorien, was read by his great-niece Iris Hunt.
It included the heartbreaking lines: "I have not heard from him at all and the War Office can tell me nothing. They have referred me to you.
"The War Office seems to think this is rather a long time. I am very anxious as it is now 10 weeks. If anything has happened to him by this time, someone would have wrote to me."
Ms Hunt held a single white rose as she delivered the reading next to his grave - and delivered a kiss as she laid it by his headstone.
Performances were heard from a children's choir conducted by Gareth Malone, as well as a recording of a collaboration between the London Symphony Orchestra and the Berlin Philharmonic Orchestra.
As the ceremony - narrated by TV historian Dan Snow - moved to a close, William, King Philippe of Belgium, Mr Cameron, and the German and Irish presidents laid flowers at the obelisk.
The Last Post was sounded, followed by a minute's silence and the reveille, then a lone piper played, after which each of the dignitaries placed a lantern on the obelisk.
St Symphorien is where 229 Commonwealth and 284 German troops are buried, including the first and last British soldiers to die on the Western Front, and the recipient of the first VC of the war.
Within weeks of Britain declaring war on Germany, the two nations' forces clashed outside Mons, leading to some 1,600 British casualties and 2,000 German.
The land at St Symphorien was gifted to the German army by a Belgian landowner, with the proviso that both sides be treated with equal respect.
Before the ceremony, William, Kate, Harry and Mr Cameron attended a reception for guests and met relatives of those buried at the cemetery.
Helen Jones, 57, whose great-uncle George Bellamy is buried at the site, visited his grave for the first time, and met Prince Harry at the reception.
Ms Jones, from Westwoodside, north Lincolnshire, said afterwards: "It happened and it was always terrible but because there's somebody there that was part of your family, it's just so personal. It makes it real."
Earlier, William spoke at the Allies' Memorial at Cointe and said: "We were enemies more than once in the last century, and today we are friends and allies."
He said that war between the nations was now "unthinkable", but warned that recent events in Ukraine were testament that "instability continues to stalk our continent".
William and Kate were in Belgium representing the Queen, who attended a service of commemoration in Crathie Kirk in Crathie, Aberdeenshire, near Balmoral.
At 11pm on August 4 1914, Britain declared war on Germany, ushering in four years of darkness, despair and appalling tragedy.
Until the armistice was signed on November 11 1918, soldiers engaged in the bloodiest conflict the world had known.
In the UK to mark its beginning, the Prince of Wales attended a service at Glasgow Cathedral, which was followed by a wreath-laying service and marchpast at the Cenotaph in George Square.
Later Britain was plunged into darkness as lights were switched off for an hour across the country to conclude the day of ceremonies.
At 10pm, homes, workplaces, public buildings and places of worship were urged to turn off their lights and leave one light burning as a symbol of hope in darkness.
The nationwide event was designed to echo the words of then-foreign secretary Sir Edward Grey on the eve of the conflict: "The lamps are going out all over Europe; we shall not see them lit again in our lifetime."
Among the famous landmarks taking part in the Light Out event was the Houses of Parliament where the clock faces of the Elizabeth Tower served as a commemorative light. Across the capital, the London Eye, Tower Bridge and 10 Downing Street also went dark for an hour.
In the same hour, the Duchess of Cornwall joined senior politicians tonight for a service of solemn commemoration at Westminster Abbey.
Key figures present included Deputy Prime Minister Nick Clegg, former foreign secretary William Hague, Labour leader Ed Miliband and Metropolitan Police commander Sir Bernard Hogan-Howe.
Mr Clegg said: "Sixteen million people perished in World War One. It's an almost unimaginable number of people who died in a war which still shapes the world as it is today."
The service included the gradual extinguishing of candles, with an oil lamp snuffed out at the grave of the unknown warrior at 11pm - the exact hour war was declared.
In Afghanistan, personnel from all three services were joined by the US Marine Corps at Camp Bastion to mark the occasion.
Around 400 personnel congregated at the base's Vigil Site for a parade illuminated by the lights of military vehicles and the site itself, before a ceremony presided over by force senior chaplain Wing Commander Geoffrey Withers.