By Paul Reynolds
World affairs correspondent BBC News website
The US president wants to discuss ways to reduce nuclear weapons
President Obama's hopes for a world free of nuclear weapons may just be a dream.
Despite his rousing rhetoric in Prague that "we can do it", huge obstacles are in the way and even he gave himself two escape clauses.
The first was that he did not necessarily expect this to happen in his lifetime. He is 47 years old, so, given that the life expectancy in the US is about 78, that means another thirty years or more in which the goal might not be realised.
And nobody knows what might happen in that time.
Sceptics might also argue that President Obama is trying to head off criticism of the US in advance of the five-yearly review conference of the NPT which comes next year
The second, and more important get-out, was his statement that so long as these weapons exist, the US will maintain a "safe, secure and effective" arsenal.
So if just one country maintains nuclear weapons, so will the US. Otherwise that country could dominate the world. And if the US does, so will Russia and China. And the French will not want to rely on the Americans so they will keep them and the British will not want the French to be the only ones in Europe with them, so the British will keep them too.
And those are just the ones whose weapons are allowed under the Non Proliferation Treaty. Those countries outside the treaty - India, Israel and Pakistan - will take a lot of persuading that they should give up what they regard as their weapons of last resort.
And North Korea is also going to be hard to get on board. The Security Council has not even deterred it from launching a rocket today.
Sceptics might also argue that President Obama is trying to head off criticism of the US in advance of the five-yearly review conference of the NPT which comes next year.
There have been many calls for the nuclear-weapons states to fulfil their obligations under Article Vi of the treaty. Anti-nuclear activists say this commits these states to eventual nuclear disarmament, though the states themselves say it commits them only to meaningful negotiations in the first instance and to disarmament only under a general, worldwide agreement.
This is the article: "Each of the Parties to the Treaty undertakes to pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament, and on a treaty on general and complete disarmament under strict and effective international control."
Nevertheless, the president's speech is significant for the ways in which he seeks to reduce the threat of nuclear weapons. He wants to change the current attitude of despair, the sense that nothing can be done.
Britain recently announced the start of a new nuclear weapons programme
It will also make it easier for the US to argue for measures against states expanding or potentialy seeking nuclear weapons.
Even before he came to Europe, he had stopped funding for America's own nuclear weapons development, the so-called "reliable, replacement warhead." This was designed to replace current warheads which have been around for so long that scientists are getting nervous about their reliability.
He has now pledged that he will try to get the Senate to ratify the Comprehensive Test Ban treaty which has been observed only informally up til now.
He has also announced, after his meeting in London with Russian President Medvedev, that the US and Russia aim to agree a new treaty to reduce warheads and possibly delivery systems by the end of this year.
He wants stronger action against states that undermine or even leave the NPT (Iran take note here).
He wants better control of fissile material (ie material that is designed to cause a nuclear explosion) in case they fall into terrorist hands.
And he is calling a special conference on all this in the US.
The new US position will put pressure on America's partners or competitors either to make a gesture or justify their current policies.
Britain has recently announced not an end to its nuclear weapons programme but the start of a new one. It has hinted though that further reductions of its already reduced stockpile might be possible.
Both Russia and China are engaged in the modernisation of their strategic forces. But the idea that they will turn about and aim for zero is not, at the moment, realistic.
President Obama will also face opposition from his domestic critics who argue, as the Bush administration did, that it is only through strength that the US and its allies can be safe.
The Wall Street Journal said last week: "The thinking here is that somehow the American example will get Russia, as well as North Korea, Pakistan and perhaps Iran, to reject nuclear weapons.
"In fact, a US nuclear arsenal that is diminished in both quantity and quality would be an incentive for these countries to increase their nuclear inventories, since the door would suddenly be opened to reach strategic parity with the last superpower."
It is not going to be easy.
STRATEGIC NUCLEAR WARHEADS AROUND THE WORLD
All numbers are estimates because exact numbers are top secret.
Strategic nuclear warheads are designed to target cities, missile locations and military headquarters as part of a strategic plan.
Israeli authorities have never confirmed or denied the country has nuclear weapons.
The highly secretive state claims it has nuclear weapons, but there is no information in the public domain that proves this.
The International Atomic Energy Agency reported in 2003 there had been covert nuclear activity to make fissile material and continues to monitor Tehran's nuclear program.
US officials have claimed it is covertly seeking nuclear weapons.