What's Obama's nuclear goal?
This summit is just one part of a drive by President Obama to demonstrate that he and the United States are in the lead in an ambitious, some would say grandiose, quest to reduce and eventually completely eliminate nuclear weapons.
Those who have spoken to the president about this say it is a real and fundamental passion. But there are a number of ways to read what he is up to and why.
It could be the pursuit of a long-held radical dream. His biggest admirers and detractors might agree that the campaign for nuclear disarmament was a core belief of the left in the 60s and 70s, and might believe that he is trying to make that dream a reality.
It could be pragmatic. He's right that terrorists setting of a bomb in a major city would change international politics for ever. So much so, that it is hard to imagine how the world would or could react.
The idea of an unfriendly country using a nuclear missile against a US ally is not a lot better. It is clearly something no president would want on his watch.
Cynics might see it as a carefully clothed exercise designed to reinforce US dominance and the status quo. After all, his plan, if successful, would remove nuclear weapons from the hands of the enemies of the US, move a lot of weapons-grade material to America and leave the US with more nukes than anyone else.
At any rate his plan has three prongs. he first and easiest was the Start treaty, the agreement that Russia and the US would reduce their vast stockpile, still leaving them far more weapons than they could possibly use.
The second, the purpose of this summit, is making sure that terrorists cannot make a weapon by getting hold of nuclear material that is poorly secured.
President Obama told the conference that this problem could only be solved by nations coming together with a new mind set.
"Just the smallest amount of plutonium - about the size of an apple - could kill and injure hundreds of thousands of innocent people. Terrorist networks such as al-Qaeda have tried to acquire the material for a nuclear weapon, and if they ever succeed, they would surely use it. Were they to do so, it would be a catastrophe for the world - causing extraordinary loss of life, and striking a major blow at global peace and stability. In short, it is increasingly clear that the danger of nuclear terrorism is one of the greatest threats to global security - to our collective security."
He went on to say it was an irony that the risk of a nuclear confrontation between nations had gone down, but the risk of a nuclear attack had gone up.
Still, his third and trickiest challenge is dealing with those nations which the US believes pose a threat because they have, or are apparently trying to get, nuclear weapons.
He announced South Korea would hold the next such summit. The country is next door to North Korea, a nation which has not been invited to this gathering because it has its own nuclear weapons programme in defiance of UN resolutions.
More pressing still is what to do about Iran. The West believes Iran is bent on getting its own bomb and wants new sanctions.
After a meeting between President Hu of China and President Obama, the White House said China was working on sanctions. But a spokesman for the Chinese foreign minister said pressure and sanctions could not fundamentally solve the problem.
This is the president's biggest challenge as there is nothing obvious that can be done to solve the problem. After all, the two previous US presidents couldn't find a solution.
You could argue the Iraq war was all about sending a message to Iran and North Korea. If so, they didn't get it. Neither country appears willing to change its approach. Military strikes set back programmes, but don't eliminate them. I can hardly imagine the world has much appetite for two new wars and occupations.
Many experts believe the world will have to learn to live with a nuclear Iran, just as it appears to be living with a nuclear North Korea. But it wouldn't do much for a president who has made a less nuclear world a top priority.
I’m Mark Mardell, the ´óÏó´«Ã½'s North America editor. These are my reflections on American politics, some thoughts on being a Brit living in the USA, and who knows what else? My 
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