By Paul Reynolds
World Affairs correspondent, BBC News website
If the current diplomatic efforts to get Iran to suspend its nuclear fuel enrichment activities do not work, it is inevitable that at some stage, attention will be turned to discussion of a military option.
The US has the military capability to attack Iran's nuclear facilities
That means, in practice, an air attack against Iran's nuclear facilities by the United States and/or Israel.
The US could certainly carry out such an attack, with cruise missiles and with B-2, other Stealth bombers and B-52 bombers armed with satellite guided bombs.
However Iran's nuclear plants are widely spread out and one is buried deep underground, so an attack would need to be sustained and wide-ranging.
Israel might also be able to do it. Not long ago it bought some bunker-busting bombs from the US, but it would be much more of a challenge.
Nobody involved in the diplomatic round says this is an active proposition at the moment.
However, President Bush has stated that the US will not accept Iran as a nuclear-armed state.
Hawk John Bolton has said the US will use all tools at its disposal
It is possible that he will interpret Iran's programme as a threat, even though Iran says it will not build a bomb but wants the technology only to make fuel for civil nuclear power. It is allowed to do make its own fuel under the Nuclear Non-Proliferation Treaty (NPT).
Vice-President Cheney said last year that Israel might act first and "let the rest of the world worry about picking up the diplomatic pieces afterwards".
And one of the administration's leading hawks, John Bolton, the US ambassador to the UN, warned Iran recently of "painful consequences" and of using "all tools at our disposal" if its nuclear programme was not stopped.
It was perhaps significant that Mr Bolton was speaking at a meeting of Aipac, the American Israel Public Affairs Committee. This is a powerful lobbying group and its priorities are closely watched to see their effect on US policies. At the moment, one of Aipac's priorities is Iran.
In the final analysis, the US might face what is being called the McCain moment. This is what Senator John McCain said: "There is only one thing worse than the United States exercising a military option. That is a nuclear-armed Iran."
However, the dilemma might be more difficult than that because Iran might not became "nuclear-armed". It might simply become nuclear-capable.
The technology in question can be used for both civilian and military purposes.
If Iran does not go down the military road, and it says it will not, there will be many governments around the world who will argue that it should be allowed to enrich fuel, under inspection by the International Atomic Energy Agency (IAEA).
The timetable is uncertain but an assessment by the Institute for Science and International Security (ISIS) in Washington says that Iran might be able to assemble enough centrifuges by 2007, and enrich significant amounts of fuel by 2008.
Those could be the red lines for the US and Israel. If Iran chose to do so, and it says it will not so choose, it could be in a position to build a bomb by 2009 or 2010, according to this assessment.
Iraqi reactor raid
So would the US agree to enrichment or would it attack? Or would it concentrate on encouraging a change of government and policy in Iran and marshalling its allies into imposing sanctions?
The UN Security Council as things stand is unlikely to do much in the way of sanctions, given Russian and Chinese opposition.
Both the US and Israel have probably made contingency plans for an attack. That would be no surprise. It is what the military does in many situations.
In June 1981, the Israeli air force bombed the Iraqi nuclear reactor at Osirak, south of Baghdad, and that raid is often used as the example of what might happen this time.
However, the raid illustrates both the feasibility of such a raid and its longer-term drawbacks.
The raid did indeed destroy the plant but it also spurred Iraq to develop a nuclear capability in secrecy - and it nearly succeeded.
The Israelis can argue that they achieved a delay that proved crucial. But history might not repeat itself.
Iran might, for example, simply leave the NPT, as it has the right to do, and go ahead with nuclear development anyway. That could set the scene for further attacks over a long period of time.
Iran might also retaliate, against US interests in Iraq and the Gulf, and might use the militant group Hezbollah in southern Lebanon to attack Israel. The region could be in uproar.
For all these reasons, and no doubt also because of the pressing US preoccupation in Iraq, the military option has not come to the fore.
There is a small group of experts and analysts, however, who think that it will come.
An article by veteran military watcher Seymour Hersh in The New Yorker in January 2005 helped lead the way.
He quoted a "former high level intelligence official" as saying: "Next, we're going to have the Iranian campaign. We've declared war and the bad guys, wherever they are, are the enemy."
Iran has a number of nuclear installations in operation
The "war" would not be an invasion of Iran but subversion leading, it would be hoped, to regime-change and an air attack if necessary. Mr Hersh indicated he felt such a subversion effort had already begun.
On this side of the Atlantic, Dan Plesch, Research Associate at the London School of Oriental and African Studies, is proclaiming the same message.
He gave a speech analysing the options recently and told the BBC News website: "The United States has the capability to come out of the clear blue sky and destroy the Iranian military infrastructure."
He went on: "You can say we are being hysterical and are a band of doom-mongers. But I fear the US has lost confidence in the UN or the EU to solve this. And it could do it militarily.
"It has reorganised its strategic forces in a doctrine known as Global Strike, meaning that from a standing start it can strike anywhere in the world in a short time. That gives it the capability."
All this does not mean it will happen. It does mean it is being debated.
Of course, the legality of any attack would be hard to justify. The British Foreign Secretary Jack Straw told reporters this week: "I don't happen to believe that military action has a role to play in any event. We could not justify it under Article 51 of the UN charter which permits self defence."
In the absence of Security Council approval, the US might argue that its interests in the Gulf were at stake and that its ally Israel was at risk.