THE Duke of Cambridge marked the 100th anniversary of Britain's entry into the First World War and said: "We were enemies more than once in the last century, and today we are friends and allies."
William saluted those who died in the Great War to give the world freedom as he attended the first of a series of commemorations in Belgium.
Delivering a speech in Liege, he said that war between the nations from 1914 to 1918, claiming the lives of millions, including 750,000 British and Commonwealth troops, was now "unthinkable".
But he warned that recent events in Ukraine were testament to the fact that "instability continues to stalk our continent".
William was joined by wife Kate at the Allies' Memorial at Cointe. The duchess, who wore a cream coat dress and pale hat, was seen chatting to French president Francois Hollande before the ceremony started.
Ireland's president Michael D Higgins and Belgium's King Philippe and Queen Mathilde also attended.
William told the guests: "The peace that we here enjoy together as allies and partners does not simply mean no more bloodshed - it means something deeper than that.
"The fact that the presidents of Germany and Austria are here today, and that other nations - then enemies - are here too, bears testimony to the power of reconciliation.
"Not only is war between us unthinkable, but former adversaries have worked together for three generations to spread and entrench democracy, prosperity and the rule of law across Europe, and to promote our shared values around the world.
"We were enemies more than once in the last century, and today we are friends and allies. We salute those who died to give us our freedom. We will remember them."
As part of the ceremony, a 10-year-old girl released a white balloon as a sign of peace and reconciliation. At the same time thousands of other balloons in the colours of the flags of the countries invited to the commemoration were also released.
German president Joachim Gauck said it was "unjustifiable" for Germany to have invaded Belgium, adding that nationalism "bonded almost everyone's hearts and minds".
He added: "We are grateful to have been able to live together with peace for so long in Europe."
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, millions of lives were lost, including 750,000 British and Commonwealth soldiers, in what was the bloodiest conflict the world had known.
Today, as part of a national day of commemoration, events marking the anniversary of the start of the Great War were held in London, Glasgow and Belgium - starting a four-year Government-led programme of remembrance.
In Glasgow, Prime Minister David Cameron, First Minister Alex Salmond and the Prince of Wales were among guests at the city's cathedral to mark the start of the conflict.
And in Folkestone, Kent, Prince Harry unveiled the seaside town's memorial arch and laid a wreath at the war memorial.
Speaking outside Glasgow Cathedral Mr Cameron said it was important to find new ways of bringing the experiences of those involved in the conflict to life, saying Britain entered the war because "there were important principles at stake".
This evening William, Kate and Harry will be among 500 guests at St Symphorien, where 229 Commonwealth and 284 German troops are laid to rest, including the first and last British soldiers to die on the Western Front.
The event will mainly be narrated by historian Dan Snow and will include readings, music and poetry capturing the history of the site.
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.
And in London at 10pm - an hour before war was officially declared 100 years ago - a service of solemn commemoration will be held at Westminster Abbey, with key figures including the Duchess of Cornwall, Deputy Prime Minister Nick Clegg, 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."
Mr Miliband said: "Young men from across Britain served alongside soldiers from across the world - from the Indian sub-continent to Africa, from Australia to the Caribbean.
"We must also remember those who served their country in other ways, from nurses who risked their lives on the Western Front to those who played their part on the Home Front."
The service will include the gradual extinguishing of candles, with an oil lamp extinguished at the tomb of the unknown soldier at 11pm - the exact hour war was declared.
In the same hour, the nation has been urged to switch off lights in places of worship, public buildings, workplaces and homes, and leave one light burning as a symbol of hope in darkness, in a reference to then-foreign secretary Sir Edward Grey's famous remark on the eve of the outbreak of war that the "lamps are going out all over Europe".