Nuclear technology is now so widespread that it is only political will which stops many countries from making nuclear weapons.
By Paul Reynolds
BBC News Online world affairs correspondent
Iran denies it wants to build nuclear weapons
Mohamed ElBaradei, head of the UN's nuclear regulatory body the International Atomic Energy Agency (IAEA), said recently that 40 countries could make the bomb if they wanted to.
The reason for this is that the technology legally used to enrich uranium to make fuel for nuclear power can easily be developed to make material for nuclear weapons.
A country could do this in secret or withdraw from the Nuclear Non-Proliferation Treaty (NPT), and do it anyway.
This is the Achilles' heel of the NPT - an agreement designed to stop the spread of nuclear weapons while allowing countries access to nuclear power.
But if even only one or two of them go nuclear, or are thought to be doing so, it could bring tension and even war into their regions.
The United States has not ruled out the use of military action to prevent proliferation.
The US Under Secretary for Arms Control and International Security John Bolton wrote in the Financial Times earlier this month:
"We are determined to use every resource at our disposal - using diplomacy regularly, economic pressure when it makes a difference, active law enforcement when appropriate and military force when we must."
Such a policy can be expected to continue under a second Bush administration. A President Kerry would probably be more cautious about the use of force.
Take Iran and North Korea, the two countries currently in the frame.
Iran says that it intends to enrich uranium to make fuel, claiming its right to do so. It is defying a demand from the IAEA for it to suspend its plan and await fuller inspections.
The US and others, including Britain, demand that Iran abandon enrichment altogether on the grounds that it cannot be trusted.
If Israel thought that Iran was using its enrichment capability to build a bomb, which Iran says it is not, it might attack Iran's nuclear facilities. Israel will certainly not give Iran the benefit of any doubt.
Only this week, reports emerged that the US was supplying Israel with 500 "bunker-busting" bombs which would be useful in any such attack. Israel has already started a diplomatic and media campaign to publicise its fears of Iranian intentions.
North Korea has withdrawn from the NPT and is said by British Foreign Office Minister Bill Rammell, who visited the country recently, to have produced possibly two nuclear devices already. Talks have so far failed to make it change its mind.
North Korea says it needs nuclear weapons to check US aggression
"A North Korean nuclear weapon could tip Japan and South Korea into making their own," said Dr Gary Samore, Senior Fellow for Non-Proliferation at the International Institute for Strategic Studies in London and a former official in the Clinton administration.
It would also force a second Bush administration to decide whether to keep talking, to reluctantly accept a nuclear North Korea and impose sanctions, or try to destroy its nuclear plants.
The risk of that is great. It could start a general war on the Korean peninsula.
Tightening the Treaty
There is a move afoot to tighten the NPT which is reviewed every five years. The next review is in 2005.
The Bush administration has proposed a number of Treaty amendments, the most important of which would stop the spread of enrichment technology.
"The first proposal would close the loophole in the Treaty that allows states such as Iran and North Korea to pursue fissile material for nuclear weapons under peaceful cover.
"Enrichment and reprocessing plants would be limited to those states that now possess them," John Bolton told a nuclear conference earlier this year.
Another proposal would prevent the sale of nuclear fuel to countries without a rigorous inspection regime.
However, Washington is not relying on the NPT being made to work more effectively.
It has initiated a much more active campaign which it calls "counter-proliferation."
It has formed the "Proliferation Security Initiative" with like-minded countries.
Sometimes called an "action not an organisation", the PSI is aimed at disrupting the sale and shipments of nuclear components, if necessary by interceptions at sea.
The US has also got the Security Council to pass Resolution 1540 which insists that member states tighten procedures to try to stop what are called "non state actors" i.e. rogue scientists from selling their wares and expertise.
The A Q Khan network
One such rogue scientist was Dr A Q Khan, the "father " of the Pakistani bomb, who was found to be transferring his expertise, certainly to Libya and possibly to Iran.
Pakistan began its nuclear programme in the 1970s
An interception of some his equipment on the way to Libya took place last year when a charter ship was diverted to an Italian port.
Libya subsequently renounced its secret nuclear programme and has been rewarded by the lifting of sanctions.
Libya is now held up as an example of how a rogue nuclear state can be brought back into the international fold.
Non-nuclear and strongly anti-nuclear countries like New Zealand point to two further flaws in the NPT.
They complain that the nuclear powers accepted as such under the NPT (the US, Soviet Union (now Russia), China, Britain and France) have not worked for total nuclear disarmament as they are supposed to and as they re-committed themselves to at the last NPT review meeting in 2000.
This leads to claims that the NPT is a club used by the powerful, especially the US, to keep down the weak.
The other flaw is that a number of nuclear powers are not members of the NPT. These are Israel, India and Pakistan. They are therefore free of restrictions. Iran for one says that this unfair and that Israel should be forced to give up its nuclear weapons.
Israel in turn claims that it is in special peril.
India and Pakistan argue that if the US and others have weapons for defence and proclaim the value of the nuclear deterrence, then so should they.
However, the failure to bring them into the NPT has tempted others to join them outside. North Korea has done so.
However, there have been non-proliferation successes.
South Africa and Ukraine gave up their nuclear weapons. The whole of South America remains nuclear-free.
"The number of countries in the NPT which have pursued nuclear weapons is very small. Libya has given up. North Korea has left. That leaves the question of Iran, " said Dr Samore.
"Statistically, the treaty is doing OK," he said.