To the British Parliament seated at majestic Westminster Hall, Obama declared: "The time for our leadership is now."
"Even as more nations take on the responsibilities of global leadership," Obama said, "our alliance will remain indispensable to the goal of a century that is more peaceful, more prosperous and more just." "After a difficult decade that began with war and ended in recession, our nations have arrived at a pivotal moment once more," he said.
Obama was granted the honour of being the first US president to speak from the grand setting of Westminster Hall, and he received a deeply friendly welcome. He recounted a history between two countries an ocean apart that was conceived in war but matured into an indispensable global force for economic growth, security, democracy and peace.
His speech came not long after Obama stood with Prime Minister David Cameron in promising jointly to continue a relentless and punishing campaign against Muammar Gaddafi's forces in Libya, saying there "will not be a let-up" in pressure to force Gaddafi out.
At a news conference with his British counterpart, Obama ruled out a deadline for ending NATO's military assault but said it would be over "in a timely fashion."
"I believe that we have built enough momentum that, as long as we sustain the course we're on, he will step down," the president said of Gaddafi.
There was unity on Libya, but the news conference exposed a split between the two leaders on the Middle East peace process. Cameron backed Obama's recently articulated call for a peace process to restart on the basis of Israel's pre-1967 borders with mutually agreed land swaps. But while Obama strongly condemned the Palestinians' plan to unilaterally seek statehood recognition from the United Nations, Cameron said it wasn't the right time to weigh in on that.
After the news conference, Obama spoke to both houses of Parliament gathered in the cavernous 11th-century Westminster Hall where generations of rulers have held coronation banquets and where many others lay in state while awaiting burial.
His address came midway through a four-country European tour during which he's connected with his unlikely Irish roots and enjoyed the hospitality of Queen Elizabeth II, even while keeping an eye on events at home where casualties are mounting from a virtually unprecedented monster tornado in Missouri.
The president spoke to grave global challenges of war, peace and economic strain, calling on Britain and the US to meet the issues head-on and together, and more broadly he exhorted the world to capitalize on momentum toward democracy and universal rights.
As revolutions sweep the Middle East and North Africa, NATO forces bombard Libya, and US and British troops fight in Afghanistan, Obama gave a robust defence of the need for the US and Britain to put their military might at the service of people around the world seeking freedom.
"If we fail to meet that responsibility, who would take our place?" the president asked.
"Our action - our leadership - is essential to the cause of human dignity. And so we must act - and lead - with confidence in our ideals, and an abiding faith in the character of our people, who sent us here today."
The president received a sustained, standing ovation for his defence of the American-British alliance and how the two "enduring allies" will be a defining force for good in the future, as well.
"As two of the most powerful nations in history, we must always remember that the true source of our influence hasn't just been the size of our economy, the reach of our military, or the land that we've claimed," the president said. "It has been the values that we must never waver in defending around the world - the idea that all human beings are endowed with certain rights that cannot be denied."
The president also got applause when he noted that it was possible for the grandson of a Kenyan who was a cook in the British army to stand in Westminster Hall as president of the United States.
And even on foreign soil, Obama sprinkled his speech with promotions for his political agenda at home. He called for spending on education and science even during austere times; for more progress on reducing the emissions that cause global warming; and for a government assurance that people can get health care.
Here, too, he said the United States and Britain can serve as model for emerging giants such as China, India, Brazil and others.
"The successes and failures of our own past can serve as an example for emerging economies - that it's possible to grow without polluting; that lasting prosperity comes not from what a nation consumes, but from what it produces, and from the investments it makes," he said.
After the speech, in a move rarely deployed in British politics, Obama worked the audience for almost 10 minutes, surprising lawmakers as he stopped for a series of brief chats.
