President Barack Obama begins a visit to Britain where he and Prime Minister David Cameron will review NATO action to help end conflict in Libya and Western policy towards uprisings in the Arab world.
He and Cameron will also launch a joint US-British group to tackle national security challenges together.
As a precautionary measure Obama flew to London earlier than expected because of fears that a volcanic ash cloud from Icelandic could drift over Ireland and prevent his Air Force One jet from flying.
That forced him to cut short a cheerful visit to Ireland, where he sipped a pint of Guinness in the village of Moneygall to celebrate ancestral roots there and used a speech in Dublin to lift Irish spirits bruised by a severe economic downturn.
In London, Obama will experience some of the celebratory pomp put on by the royal family while on a four-nation trip that will include a Group of Eight summit in Deauville, France, later in the week and conclude with a stop in Poland.
On Tuesday he will attend a formal arrival ceremony at Buckingham Palace laid on by the Queen, a wreath-laying at Westminster Abbey, talks with Prime Minister David Cameron and a state dinner hosted by the Queen. Obama and his wife, Michelle, are to spend two nights at Buckingham Palace.