US President Barack Obama officially started a state visit to Britain today complete with pomp and splendour and the glamour of a meeting with Prince William and his new wife Catherine.
Obama and his wife Michelle arrived in Britain late yesterday, leaving Ireland early to avoid the Icelandic volcanic ash cloud, and spent the night at the US ambassador's residence in London.
Today they were formally welcomed to Buckingham Palace, where they will stay in the sumptuous Belgian suite, last used by William and the former Kate Middleton on the night of their April 29 wedding.
The rest of the three-day visit will mix diplomacy and ceremony, including a state banquet in their honour and an informal barbecue for British and US servicemen.
Setting the tone for their talks during the visit, Obama and British Prime Minister David Cameron vowed today they would not abandon the demonstrators pushing for democracy in the Arab world.
"We will not stand by as their aspirations get crushed in a hail of bombs, bullets and mortar fire. We are reluctant to use force, but when our interests and values come together, we know we have a responsibility to act," they wrote.
The declaration of intent came after Obama last week gave his long-awaited response to the tumult which has ousted autocrats and reshaped nations in the Middle East and North Africa.
In their article, the two leaders added: "We will stand with those who want to bring light into dark, support those who seek freedom in place of repression, aid those laying the building blocks of democracy."
Giving a new twist to the so-called "special relationship" between the United States and Britain, Obama and Cameron also heralded a new "essential relationship" between the countries.
Britain is reportedly planning to send Apache helicopters to attack Muammar Gaddafi's forces in Libya, in a bid to unlock the stalemate there.
Obama is to be welcomed by Cameron at his Downing Street residence in the afternoon before the formal talks on Wednesday, which come before both men attend the G8 summit in France on Thursday and Friday.
Obama will also visit Westminster Abbey where he will lay a wreath at the Tomb of the Unknown Soldier.
But the focus of today's programme is the royals, and especially the meeting with William and Catherine, fresh from their honeymoon in the Seychelles.
The two couples met in the lavish 1844 room at Buckingham Palace for about 10 minutes just before noon, before the formal ceremonial welcome for the Obamas in the palace garden.
The Obamas were not invited to the wedding because William is not heir to the throne -- he is second in line after his father, Prince Charles -- so the meeting was the president's first chance to congratulate the couple in person.
After the formal welcome and lunch, Queen Elizabeth II will show the Obamas US-related items in the Royal Collection and the Royal Archives at the palace.
The Queen struck up a visibly close friendship with Michelle Obama when the Obamas visited Britain for the first time in 2009, with both women putting their arms around each other in a highly unusual gesture.
Michelle and her two daughters have since made a private visit to the palace.
Tomorrow, the president will be granted the rare honour of addressing both houses of the British parliament.
The week-long European visit started in Ireland yesterday, where the Obamas received a rapturous welcome and the president downed a pint of Guinness in the tiny town of Moneygall, the home of his great-great-great grandfather.
In Dublin, Obama delighted a crowd of 25,000 people, telling them he was "Barack Obama of the Moneygall Obamas" and insisting Ireland had a bright future despite the current economic turmoil.