The 2009 TIME 100

I'm not betraying state secrets if I describe the last moments of the G-20 summit in london. It was Barack Obama's first summit as President. He was — with one exception — the youngest leader there, the newest in office, and there was the ever present risk of America being seen as lecturing the world rather than leading it. Softly and with a genuine humility, he talked about how we could together replace people's fear of the future with a new faith in the future, and how we could look past the storm to the great potential that lies beyond it. The next day his calm persuasion turned a potential contest for the position of NATO Secretary-General into an appointment by unanimous acclaim. That week, Europe and the U.S. exchanged ideas and shared ways of reshaping the financial system, achieving nuclear disarmament and nonproliferation and tackling the crucible of terrorism on both sides of the Afghanistan-Pakistan border.

Time and time again, people have talked of Barack Obama's talent for listening. His real talent is for hearing what is actually said. His rare combination of idealism and realism struck me when I first met him: a natural ability to lead, combined with the qualities of mind and spirit that always seek to reach out and connect.

As the first black President, Obama, 47, has already proved that once inaccessible pinnacles can be reached. His swift and decisive action on the economic crisis has been impressive in itself but is only one pioneering achievement of his first hundred days. Now global problems need global solutions, from our bold initiative to give every child in every country the opportunity of a good education, to our shared ambition of an agreement on climate change at Copenhagen in December. Obama is working with world leaders to take on the unparalleled challenges of the global age: in development, climate change, energy, terrorism and security.

Of course, his oratory is today unmatched. But his courage — the courage to go first, to lead, where none have gone before — is doubly unmatched. When he speaks, he gives those who hear him confidence: not in him but in themselves. It was said of Cicero that when people heard him, they turned to one another and said, "Great speech"; but when Demosthenes spoke, people turned to one another and said, "Let's march." All around the world people are marching with Barack Obama.

Brown is the Prime Minister of the United Kingdom

Fast Fact: The hoops-loving President installed a backboard on the White House tennis court