Britain and America's shared values will continue to shape the world, Barack Obama has told the UK's lawmakers.
The first address by an American president in parliament's Westminster Hall saw Mr Obama address the question of how to maintain western influence as new economic powers including China and India emerged.
"Today, after a difficult decade that began with war and ended with recession, our nations have arrived at a pivotal moment once more," he warned.
"The time for our leadership is now. Even as more nations take on the responsibilities of global leadership, our alliance will remain indispensable to the goal of a world that is more peaceful, more prosperous and more just."
The United States president warned that the US and UK face "profound challenges" at a "pivotal moment" in the history of the world.
As the countries whose commitment to "market-based principles" had helped cause globalisation, Mr Obama argued, Britain and America had an inherent advantage in the struggle to keep their power.
"We will have to redouble our investments in science and engineering and renew our national commitments to educating our workforces," he said, if the US and UK are to continue their pre-eminence in science, "cutting-edge research" and the discovery of new medicines and technologies.
After a joint press conference with prime minister David Cameron in which the pair acknowledged their new approach to foreign policy was one based on pragmatism, Mr Obama went a step further in his speech by claiming the US would proceed with "humility".
Interventions would take place nonetheless, he said. "From the beaches of Normandy, to the Balkans, to Benghazi - that is our interests and our ideals," Mr Obama said.
"As two of the most powerful nations in the history of the world, we must always remember the true source of our influence hasn't just been the size of our economies, or the reach of our militaries, or the land that we claim. It has been the values that we must never waver from defending around the world," he added.
"We are the nations most willing to stand up for the values of tolerance and self-determination that lead to peace and dignity.
"We must act and lead with confidence in our ideals and an abiding faith in the people who set us all here today.
"For if I believe there is one final quality that makes the US and the UK indispensable in this moment in history, it is how we define ourselves as nations."
Rather than finding division in ethnicity, he argued, both Britain and America distinguished themselves by embracing diversity.
"If they work hard, they can pledge allegiance to our flag and call themselves Americans," he added.
"If they come to England they can make a new life for themselves and sing God Save The Queen just like any other citizen.
"Yes, our diversity can lead to tension... but we fundamentally recognise our patchwork heritage is an enormous strength."
Mr Obama received continuous applause as he slowly worked down Westminster Hall, shaking hands with appreciative parliamentarians.