President Barack Obama today warned of profound challenges ahead for Britain and the US.
In a keynote speech at Westminster Hall, he said the relationship between the two countries was "one of the oldest and one of the strongest" alliances the world has ever known.
"There are few nations that stand firmer, speak louder and fight harder to defend democratic values around the world than the United States and the United Kingdom," said the president, who is on a two-day state visit to Britain.
"We are the allies who landed at Omaha and Gold, who sacrificed side by side to free a continent from the march of tyranny and helped prosperity flourish from the ruins of war. With the founding of Nato - a British idea - we joined a transatlantic alliance that has ensured our security for over half a century."
However, he warned of challenging times ahead. "In a world where the prosperity of all nations is now inextricably linked, a new era of co-operation is required to ensure the growth and stability of the global economy.
"As new threats spread across borders and oceans, we must dismantle terrorist networks and stop the spread of nuclear weapons, confront climate change and combat famine and disease. And as a revolution races through the streets of the Middle East and North Africa, the entire world has a stake in the aspirations of a generation that longs to determine its own destiny."
The times were gone, he said, when Roosevelt and Churchill could "solve the world's problems over a glass of brandy" - adding, to laughs from the hall: "I'm sure that Prime Minister Cameron and I would agree that some days we could both use a stiff drink."
Mr Obama and his wife are hosting the Queen and Prince Phillip at dinner this evening in Winfield House - the residence of the US Ambassador.
Earlier, both men warned Libya's Muammar Gadafy there would be no let up in pressure on him to go but said it will require a slow and steady campaign.
The two leaders met against a backdrop of a stalemate in the three-month conflict, with Col Gadafy hanging on to power despite a Nato air campaign launched to protect civilians and attack Libyan government targets.
"I believe that we have built enough momentum that as long as we sustain the course that we are on that he is ultimately going to step down," Mr Obama said. He also defended US policy of backing off the air campaign after initially leading the effort, a policy that has led to some criticism from US allies in Europe.
Britain, France and other Nato members have taken the lead but face just as many financial hardships at home as the United States, and there are some suggestions that the allies would like Washington to do more.
"Ultimately this is going to be a slow, steady process in which we are able to wear down the regime," Mr Obama said.
Mr Obama, who was feted by Queen Elizabeth at a state dinner in London last night, moves to France tomorrow for a Group of Eight summit in Deauville and concludes his week-long trip in Poland on Friday and Saturday.
Mr Obama and Mr Cameron went out of their way to gloss over any differences between the two allies, and in a show of bonhomie, they served up grilled burgers and sausages to US and British soldiers and their families in a shirt-sleeved appearance at No 10 Downing Street.
While Britain has opted for stern austerity measures to deal with a debt crisis, the Obama administration has resisted deep spending cuts as his Republican opponents demand.
In the name of unity, all the two leaders could say on this obvious difference between them was that each country will have to deal with problems in its own way. "I completely agree with Barack that each country is different," said Mr Cameron.
Mr Cameron said he supported Mr Obama's move last week to declare Israelis and Palestinians should open peace negotiations based on the lines that existed before the 1967 Six Day War in the Middle East, a policy move that has enraged Israeli prime minister Benjamin Netanyahu. Mr Obama stuck to the position, saying it is going to require "wrenching compromise" on both sides.
Reuters