By Paul Reynolds
World Affairs correspondent, BBC News website
The debate over whether the US might attack Iran's nuclear installations has taken a step forward with the publication of an article which suggests that the US has considered using a nuclear "bunker buster" in such an operation.
The US has the military capability to attack Iran's nuclear facilities
The article, in The New Yorker magazine, is by veteran military analyst Seymour Hersh.
It is a follow-up to one he wrote in January 2005 in which he suggested that an air attack on Iran was possible. What is new is that he raises the prospect that such an attack might be nuclear in nature.
However, the problems of using a nuclear weapon would be so huge that few people are taking this seriously. The British Foreign Secretary Jack Straw said the idea was "completely nuts".
Hersh himself downplayed the prospect. In an interview with the BBC, he said the Pentagon had told the Bush administration initially that a nuclear attack was the only way of guaranteeing success:
"Nobody was advocating it, they were just saying a 100% guarantee. Where it becomes interesting, the joint chiefs, in one of its subsequent papers, wanted to withdraw that option because of course it's madness, a nuclear weapon in the Middle East to an Arab [sic] Muslim country, my God. And the White House won't withdraw.
"That's the issue, that the White House, some people there still wanted to have this option. That's what's causing the trauma, not that they're going to do it, but the White House won't take it off the table."
One should separate the two issues here. The first is a possible attack on Iran. The second is the nature of that attack.
An attack at some stage is possible and the White House has not ruled anything out. It never does, in fact. For the moment, though, diplomacy is the chosen route in the effort to get Iran to give up its nuclear programme. This explains why western diplomats talk about an attack not being "on the agenda." It is not at present; but the phrase does not rule it out for the future.
Nuclear option problems
But a nuclear attack is improbable.
There are three main reasons of military and diplomatic importance for this.
The first is that even a nuclear "bunker-buster" would produce large amounts of radiation. This could cause thousands of casualties among civilian populations.
The Federation of American Scientists says that "the bombs would penetrate at most only a few metres into rock, causing no reduction in blast, fire, or fallout damage on the surface. The largest would have blown out a crater almost a thousand feet across and thrown a cloud of radioactive fallout tens of thousands of feet into the air where it would be blown hundreds of miles downwind."
The second is that the political implications are so huge of the US attacking, with nuclear weapons, a country (and in the Muslim world) which is not armed with similar weapons and which says it has no intention of making.
The third reason is that, doctrinally, the US is moving away from developing new nuclear bunker busters. It does have one already, the B61-11, but it cannot penetrate very deeply and last year Congress withdrew, at the administration's request, funding for further research.
There is also the question of legality to be considered.
Any attack would be hard to justify. Jack Straw told reporters recently: "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.
Instead of nuclear bombs, conventional weapons would be used, but of a massive type. They would try to do a similar job, but without the same physical and political fallout. One called "Big Blu" is currently under development.
Hersh also said, in both articles, that the US had infiltrated agents into Iran to pick targets and make contacts with dissident groups.
Iran insists its nuclear programme is for peaceful purposes
One reading of the White House attitude is that it wants to scare Iran into making concessions and wants in any case to foment political unrest. This it hopes might eventually produce policy change by producing regime change. Hersh says that Washington regards President Ahmadinejad as a "potential Adolf Hitler."
What nobody doubts is that the US is determined to stop Iran from become nuclear-armed.
Iran says it will not build a bomb but wants the technology only to make fuel for civil nuclear power. It is allowed to make its own fuel under the Nuclear Non-Proliferation Treaty (NPT).
However, the dilemma might be more difficult than that because Iran might not become "nuclear-armed". It might simply become nuclear-capable.
The technology in question can be used for both civilian and military purposes.
If attacked, Iran might 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.
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.
All this does not mean it will happen. It does mean it is being debated.
CONVENTIONAL v NUCLEAR BUNKER BUSTERS
US conventional earth penetrating weapons (EPW) the GBU-28 and GBU-37 can break through 6m of reinforced concrete or 30m of earth
Guided by laser or GPS they may also carry a 'smart fuse' which detonates the weapon at a pre-programmed point
Insufficient penetration for use against "hardened and deeply buried targets"
A nuclear EPW such as the B61-11 can only penetrate 2-3m but even a shallow underground nuclear explosion produces an intense and damaging seismic shock
Depending on the rock type, this could potentially crush a bunker at about 70 metres depth
But the missiles cannot penetrate deeply enough for the radioactive fallout to be contained