As William attended commemorations of 100 years since Britain's entry into the First World War in Belgium, he said the UK owed a great debt of gratitude to the country for its fortitude and resistance during the war.
The duke attended a ceremony in Liege, Belgium, with wife Kate to mark exactly 100 years since the country was invaded by Germany - sparking Britain's declaration of war and entry into conflict.
Over the next four years, 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.
The royal couple, who are representing the Queen, were joined by dignitaries from the countries across Europe, including France, Belgium and Germany, for the ceremony at the Allies' Memorial at Cointe, overlooking the Belgian city of Liege.
Later today they will join 500 guests including Prince Harry and Prime Minister David Cameron at a twilight ceremony at St Symphorien Military Cemetery in nearby Mons.
As part of a national day of commemoration, events marking the anniversary of the start of the Great War are being held in London, Glasgow and Belgium - starting a four-year Government-led programme of remembrance.
The royal couple were welcomed to Belgium today by the country's King Philippe and Queen Mathilde ahead of the ceremony at Cointe.
Kate wore a cream coat dress with pleated skirt and Peter Pan collar and a pale hat and took her seat between French president Francois Hollande and husband William, who wore the Queen's golden and diamond jubilee medals.
Fellow dignitaries included German president Joachim Gauck, as well as Belgian Prime Minister Elio Di Rupo, who all also delivered speeches during the ceremony.
In his speech, William described how during the war Europeans were "engulfed by killing and destruction," saying: "Among the very first victims were the people of Belgium, whose resistance was as gallant as their suffering was great."
He said: "Your great sacrifice and your contribution to eventual victory was pivotal," adding: "Many nations here today, the United Kingdom among them, owe you a great debt of gratitude for your fortitude and resistance.
"Your Majesties, if I may say so, Belgium's steadfast remembrance of your war dead, and ours, is a great credit to your nation.
"On behalf of Her Majesty Queen Elizabeth, head of the Commonwealth, thank you for the honour you do us all."
He went on: "In Europe, the transition from war to lasting peace has taken time. Edith Cavell was a British nurse who saved soldiers from each side.
"On the night before she faced a German firing squad, she said: 'I must have no hatred or bitterness towards anyone'.
"It took another terrible war to learn the truth of her words, and even today we continue to learn that lesson. The events in Ukraine testify to the fact that instability continues to stalk our continent.
"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 girl, symbolising the passing on of remembrance to future generations, 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.
The ceremony saw wreaths laid by the King of Belgium as well as a minute's silence and The Last Post.
Music included the Belgian national anthem, the Brabanconne, performed by the Belgian Royal Air Force Band and the Concert Band of the German Armed Forces.
Neutral Belgium's involvement in the First World War stemmed from an ultimatum the country was given by Germany on August 2 1914 demanding free passage for its troops into France.
Belgium rejected the demand and Germany invaded on the morning of August 4, and on the same day the first Belgian soldier, Antoine Fonck, was killed outside Liege.
Today, other key guests from the 83 countries invited to the event included Ireland president Michael D Higgins.
A film was aired showing original archive images of the invasion of Belgium, the resistance of the Belgium Army and the destruction of Loncin Fort.
Following speeches, two cannon shots were fired before the King of Belgium laid wreaths at the foot of the memorial.
King Philippe told the audience: "We are paying tribute today to the courage and dignity of those engaged in the fighting and those who lived in inhuman conditions.
"We remember also the cruelty and barbarism, healed as we are of our resentment and of the terrible wounds that affected our families.
"Finally we are expressing our gratitude to all of those who, in the depths of the darkest nights of the conflict, built up the powerful momentum of solidarity when faced with the suffering of the people and the desperate food shortages."
He added: "The memory of the First World War gives us food for thought about the responsibility of leaders and the decisions they can take to keep the peace and bring nations closer together.
"This challenge is now of major importance. The European memory reminds us that no peace can be sustained without a state of mind that overcomes the suffering endured, goes beyond the question of guilt and sets its sights firmly on the future.
"Peaceful Europe, unified Europe, democratic Europe. Peace is what our grandparents longed for with all their might."
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."
A minute's silence was held followed by the Last Post and 12 more cannon shots symbolising the resistance of the Liege forts. Between August 4 and 6, the Belgians and the Germans each lost about 1,000 lives around Liege.
The Prime Minister earlier paid tribute to those who lost their lives in the First World War, declaring it had "profoundly changed our world".
Speaking outside Glasgow Cathedral ahead of a service of commemoration for Commonwealth leaders, 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".
Earlier he said: "A hundred years ago today Britain entered the First World War and we are marking that centenary to honour those who served, to remember those who died, and to ensure that the lessons learnt live with us forever.
"It is right to remember the extraordinary sacrifice of a generation and we are all indebted to them because their most enduring legacy is our liberty."
The Prince of Wales attended the service at Glasgow Cathedral, which was followed by a wreath-laying service and march-past at the Cenotaph in George Square.
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 I. 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".