US President Donald Trump and Queen Elizabeth II gather with 300 veterans on the south coast of England on Wednesday to commemorate the 75th anniversary of D-Day - one of the heroic turning points of World War II.
Other world leaders will join them in Portsmouth for Britain's national event honouring the Allied cross-Channel invasion of the Normandy beaches in France.
It remains the largest amphibious assault in history, claiming the lives of an estimated 4,400 troops in one day.
Trump, the queen and British Prime Minister Theresa May will be joined by Canadian Prime Minister Justin Trudeau, French President Emmanuel Macron and German Chancellor Angela Merkel for an event with the veterans, the youngest of whom are now in their 90s.
It will be the last act of Trump's three-day state visit to Britain.
Portsmouth was the main staging point for some 156,000 US, British, Canadian and other Allied troops who sailed for the beaches of northern France.
The Battle of Normandy on June 6, 1944, led to the liberation of Europe and helped bring about the end of World War II the following year.
'Allies and friends'
In a joint D-Day proclamation, the 16 attending nations affirm their shared responsibility to ensure that the horrors of World War II are never repeated.
"Over the last 75 years, our nations have stood up for peace in Europe and globally, for democracy, tolerance and the rule of law," it said.
"We will work together as allies and friends to defend these freedoms whenever they are threatened."
The event on Southsea Common, an expanse of waterfront grassland in Portsmouth, includes an hour-long production recounting the story of the invasion with testimony from veterans, theatrical performances and live music, as well as a flypast.
Some 4,000 military personnel, 26 British military aircraft and 11 British naval vessels will participate, along with presidents and prime ministers from other EU nations, Australia, New Zealand.
Tragic letters
In the event's official programme, Queen Elizabeth, 93, will say: "I am sure that these commemorations will provide an opportunity to honour those who made extraordinary sacrifices to secure freedom in Europe. They must never be forgotten."
Trump will read an excerpt from the prayer President Franklin Roosevelt delivered by radio on D-Day, when he told the US nation about the Normandy operation.
May will read a letter written by Captain Norman Skinner to his wife Gladys on June 3, 1944.
The letter was in the British soldier's pocket when he landed on Sword Beach in Normandy on June 6. He was killed the next day.
Macron is expected to read the last letter of French resistance fighter Henri Fertet, who was executed when he was just 16.
"The soldiers are coming to get me. I must hurry," the letter says. "I am not afraid of death, my conscience is completely clear."
Trump sows confusion
The D-Day commemorations are among May's last official duties before she steps down as leader of the governing Conservatives on Friday.
That will trigger a leadership contest. The winner, chosen from 11 candidates, will become the next prime minister by the end of July.
May's meetings with Trump have gone smoothly. The US leader tweeted on Wednesday that he "could not have been treated more warmly in the United Kingdom by the Royal Family or the people".
He reaffirmed his commitment to a "very big Trade Deal" with Britain after Brexit
But he also sowed some characteristic confusion, walking back a series of comment he made at a joint press event with May on Tuesday.
Trump announced then that he had turned down Labour opposition leader Jeremy Corbyn's request for a one-on-one meeting, calling the socialist politician "a somewhat negative force".
But the US leader told ITV television in an interview aired Wednesday: "I didn't think it was appropriate to meet him, but I would. I certainly would have no problem with it."
He also told ITV that he did not mean to say that US pharmaceutical companies' access to Britain's state-run health care system would need to be up for negotiation in the free trade talks.
"I don't see it being on the table," Trump told ITV.
After Queen Elizabeth formally bids him farewell, Trump is off to his golf resort in Ireland before heading to further D-Day commemorations in France's Normandy region on Thursday.
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