During his first year in office, President Obama was often best overseas when he was behind a lectern or onstage before a crowd with a microphone in his hand.
But in convening his first international summit -- the largest on a single issue in Washington history -- he focused more squarely on his relationship with world leaders. He slapped backs, kissed cheeks and met one on one with more than a dozen heads of state, leavening his appeal to shared security interests with a more personal diplomacy.
The approach marked a shift for Obama as he seeks to translate his popularity abroad into concrete support from fellow leaders for his foreign policy agenda, most urgently now in his push for stricter sanctions against Iran.
"He's in charge, he's chairing the meetings, and this is where his personality plays a big part," said Pierre Vimont, the French ambassador to the United States, who compared Obama's role during the summit to the way he led the bipartisan health-care meeting at Blair House in February.
"He does it very well," Vimont continued. "And he feels very comfortable doing it."
Obama used the summit and its sidelines to elevate the arcane issue of nuclear materials security, once the province of scientists and think tanks, to a higher rank on the international security agenda.
But his achieving consensus on the goal of locking down all loose nuclear materials in four years was mitigated by the fact that participation is voluntary. Progress, or lack thereof, will be measured in two years, when leaders gather in South Korea for the second Nuclear Security Summit.
In his role as host, though, Obama gave his fellow heads of state a taste of what has been familiar to many Americans who followed the domestic political debate over the past year: the president as seminar leader.
For four hours Tuesday, Obama led a pair of planning sessions to iron out the final details of the communique that was the culmination of the summit.
He sat at the center of the gathering, calling on leaders to speak, embellish, oppose and offer alternatives to the plan taking shape. Only the heads of state and, at times, two senior aides were allowed in the room, an exclusivity some diplomats called rare.
"He's never better than when he's the teacher," said a European diplomat, who spoke on the condition of anonymity because he was not authorized to discuss the issue publicly. "Many of those who attended were just happy to be in the picture with Obama. I mean, he did get 46 leaders to Washington on a boring issue. That's pretty good."
Obama's attention to his guests began on the summit's opening night, when he spent more than an hour and a half greeting the 46 foreign leaders and three heads of international organizations he invited.
German Chancellor Angela Merkel, whom administration officials describe as high on the list of the European leaders Obama most admires, received a kiss on each cheek at the final bilateral meeting.
Obama bowed formally to Japanese Prime Minister Yukio Hatoyama. He used both hands to shake the hands of some leaders and joked with others.
David Miliband, Britain's foreign secretary, said such personal diplomacy is "quite important" at summits, especially one about an issue he said is "often seen as administrative."
"When Obama stands up and says 'My friend Dmitry Medvedev' or 'My friend Nicolas Sarkozy,' he's right, and that's important," Miliband said. "He's made a number of friends of world leaders, and I think that's a testament to why so many arrived to take part in this."
Several European diplomats said the large number of attendees reflects Obama's popularity abroad -- in some places, it exceeds his domestic approval rating -- and the fact that an appearance at a high-profile conference can often improve the stature of foreign leaders struggling with poor economies and political unpopularity at home.
But many of those leaders cared far less than Obama does about securing the military sites, research reactors and universities where nuclear materials are stored. His challenge was to change that, and he said at a news conference that he succeeded.
"Coming into this summit, there were a range of views on this danger," Obama said. "But at our dinner last night, and throughout the day, we developed a shared understanding of the risk."
On the margins of the summit, Obama met with the leaders of Canada, Jordan, Malaysia, South Africa and Turkey, among other regional powers. In all, he held at least 15 bilateral meetings, many focused on rallying support for fresh sanctions against Iran.
Among Obama's longest was with Chinese President Hu Jintao, a 90-minute session Monday that included a discussion about how to proceed with new penalties. China, a veto-holding member of the U.N. Security Council, has declined for months to endorse such measures against Iran, one of its major oil suppliers.
Afterward, the Americans asserted that China had taken a step toward endorsing stricter sanctions and reiterated the president's assessment that a new set would be in place this spring. They noted that Hu used the word "sanctions" -- a rarity for a Chinese leader -- and that Chinese officials had refrained from publicly voicing opposition to such measures.
Chinese Vice Foreign Minister Cui Tiankai, speaking to reporters in Washington on Tuesday night, said that while China prefers a diplomatic solution, it is "open to ideas" on how to deal with Iran.
Obama and Hu often take a long time to conclude a meeting, each vying for the last word. The Monday session was no different, a Chinese official said, with the two men ending and then beginning the conversation again nearly a dozen times before it was over.
Obama did not say Tuesday how soon he expects progress on Iran, saying only that China has sent representatives to the United Nations to work on a sanctions resolution.
At the news conference, Obama called the day "enormously productive" and the summit itself "historic."
"So I'm going to keep on at it," Obama said. "But I think on all these issues -- nuclear disarmament, nuclear proliferation, Middle East peace -- progress is going to be measured not in days, not in weeks. It's going to take time. And progress will be halting."
Staff writer John Pomfret contributed to this report.