The Big Question: Does the US intend to attack Iran, or is it only sabre-rattling?

Why has this question arisen now?

President Bush and other US officials have upped the anti-Iranian rhetoric alarmingly recently. The new verbal onslaught began with Mr Bush's address to the nation on 10 January this year, when he rejected the Iraq Study Group plan for Iraq, and specifically its recommendation to talk to Iran. He ratcheted up the language further in his State of the Union address last week, accusing Iran of sponsoring terrorism.

Now, the US State Department's Iraq co-ordinator is touring Europe, inveighing against what the US sees as Iran's misdeeds in the region. Reinforcing the rhetoric in the US are television adverts, depicting Iran as a nuclear menace and demanding tougher UN sanctions. What is striking is how far the emotive and intemperate language now being directed against Iran echoes the tirades that preceded the war in Iraq.

Precisely what does the US have against Iran?

There are so many issues it is hard to know where to begin. The most immediate US charges concern Iran's alleged support for Shia militants in Iraq. Washington claims that Iran is bank-rolling and arming them. The US also objects to what it sees as Iran's support for Hizbollah in Syria and Lebanon - whose strength Israel seriously underestimated when it invaded Lebanon last summer. Behind these accusations lies a greater concern: that Iran is emerging as chief beneficiary of the US failure in Iraq; and that the destabilising spread of its influence must be halted at any cost.

But there are other sources of hostility, too, aren't there?

Two other issues lurk in the background. The first, in which the US has recently been content to make common cause with the Europeans and the UN, is the fear that Iran is using a legitimate nuclear energy programme as cover for developing a nuclear weapon. The other is the deep resentment left over from the hostage crisis of 1979, when revolutionary guards stormed the US embassy in Tehran and held 52 US citizens captive for 15 months. Despite occasional overtures from both sides since, diplomatic relations have still not been restored. The election in 2005 of the anti-Western populist, Mahmoud Ahmadinejad, frustrated hopes of any early reconciliation.

Has the US yet gone beyond rhetoric?

Yes and no. Aside from the rhetoric, it is moving two aircraft carrier groups to the Persian Gulf in a show of military strength. The planned injection of 21,000 more US troops into Iraq - the so-called "surge" strategy due to take effect in the next three months - is also seen by some as a preliminary to the use of Iraq as a base for strikes on Iran's nuclear facilities, or even a more concerted assault. Others see the "surge" as directed against Iran's growing influence in Iraq. US officials partly confirm this, saying that they reserve the right to attack Iranian targets inside Iraq, but not beyond. This is what they are telling Britain and other European allies at present.

How has Iran responded?

Its priority, so far as can be judged, is not to seem to be capitulating in the face of US pressure. A rhetorician equal to anyone in the US administration, President Ahmadinejad has so far given no quarter. The Iranian response as a whole, however, has been more confusing. Iran has not made any overt retaliation for the arrest by US forces of five Iranians in Arbil, in northern Iraq. Last week, in an announcement that seemed to cock a snook at the US and the UN, an Iranian MP said Iran had taken delivery of 3,000 centrifuges for use in the enrichment of uranium. The head of Iran's nuclear energy agency, however, denied this, although there is no hint that Iran will meet UN demands for the suspension of its nuclear programme.

What might this response say about the Iranian regime?

While the US seems to be cultivating a deliberate ambiguity vis a vis Iran - speaking loudly while carrying a very small stick - Iran's response suggests that the regime is divided about how to parry US and international pressure. Crucially, Mr Ahmadinejad seems to have lost some authority. He came to office on a programme of radical economic reform intended to quell the frustration of Iran's multitude of have-nots. Almost two years on, he has failed to deliver and many of his erstwhile voters are demoralised. Exposed as weak at home, Mr Ahmadinejad cannot risk being seen as weak abroad as well. The temptation to talk big must be great. The risks associated with such a stance, however, were illustrated all too graphically by Saddam Hussein.

What are the risks for the US?

The belligerent rhetoric from Washington, combined with an insecure leadership in Tehran, creates a febrile atmosphere in which one or other side could overreact. Any strike by the US against Iranian targets in Iraq could escalate quickly into a bigger confrontation. It is not hard to imagine a scenario in which US troops pursue Iranians across the border and one thing leads to another. The lack of diplomatic relations between the two countries is an additional complication, as talks are conducted through intermediaries and liable to misunderstanding.

The official US line at this stage is that it will not attack Iran inside its borders. But whatever else is said about George Bush, he cannot be described as risk-averse. Were he to order the use of force, the stakes could be even higher than in Iraq. At worst, the US would be bogged down in another costly and probably unwinnable war; Iran would be emboldened to accelerate its nuclear programme, and the US would have to cede regional hegemony to Iran.

Where does Israel fit into this scenario?

Israel is seen by some as a power that could do Mr Bush's dirty work in Iran for him. Its 1981 destruction of Iraq's Osirak nuclear reactor is cited as the precedent. This, however, ignores the fact that Iran's nuclear facilities are widely dispersed and that Iran is more powerful than Iraq was at the time. It also presupposes that Israel's military is as effective a machine as it was then, something the Lebanon war has called into question.

It is true that Israel has taken a hawkish stance towards Iran, largely because of Mr Ahmadinejad's threat to "wipe Israel off the map". Israel's Prime Minister, Ehud Olmert, said recently that Israel would not allow the world to be "indifferent to calls for the destruction of Israel". Mr Olmert, though, is another national leader under pressure, with his own reasons for rhetorical excess.

Is there a case for a US military attack on Iran?

Yes...

* Iran has defied the IAEA and the UN; it is in clear breach of its international legal obligations

* A nuclear Iran would destabilise a region that is already highly volatile and encourage others to acquire nuclear weapons

* Iran, with its particular strain of militant Islam, is a menace not just to the US, but to wider western interests, and should be curbed

No...

* As a signatory to the Non-Proliferation Treaty, Iran is entitled to develop nuclear power; it denies it intends to build a weapon

* Iran has legitimate security and strategic interests in Iraq and is entitled to defend them

* Take one look at the disastrous consequences of US unilateral intervention in Iraq