The Bush administration has been piling up the pressure on the mullahs who rule Iran - accusing them of sponsoring terrorism, developing weapons of mass destruction and denying freedom to their people.
But does the United States favour a change in the regime's behaviour - or a change of regime?
There has been a distinct toughening of the US administration's attitude to Iran ever since President George W Bush famously declared it part of an "axis of evil" 18 months ago.
The US accuses Iran's leaders of supporting terrorism
In the eyes of this administration, the Iran of the ayatollahs - like North Korea and Saddam Hussein's Iraq - is guilty of two main things, which in Washington's view are interrelated - supporting terrorism and developing weapons of mass destruction.
And it's above all the recent evidence about Tehran's nuclear programme which has made Iran a hot issue in US policy-making circles.
Michael Eisenstadt is a military expert at the Washington Institute for Near East Policy, an influential think-tank:
"This has been a major issue for US policy-makers for at least the last decade. However, recent revelations - prompted by leaks by an Iranian opposition group, which were then confirmed by the US Government - have led people to conclude that Iran is a lot further along on its nuclear programme than previously believed.
"Some people believe that perhaps within two or three years the Iranians might have enough fissile material to produce their first bomb. On the other hand, there are US Government estimates which say that we're talking about a timeframe toward the end of the decade," he says.
Tehran denies it has a nuclear weapons programme
"So it's not clear exactly when the programme might bear fruit. What is clear is that if the programme is to be stopped, action has to be taken now," he says.
But what action? The administration is sharply divided between the so-called "realists" - who favour using concerted diplomatic pressure on Iran - and the hard-liners, known as "neo-conservatives" or "neo-cons", who favour "regime change".
One of the well-known neo-cons is Joshua Muravchik of the right-wing think-tank, the American Enterprise Institute.
He links the need for democratic change - not only in Iran but throughout the Middle East - directly to the 11 September attacks against New York and Washington.
"I think that what hit home to Americans at 9/11 was that we have to, not just retaliate, but really try to defuse this threat. And I think that the terrorism - Middle Eastern terrorism - comes out of a poisoned political culture," he says.
For the neo-cons, the tyrannies of the Middle East must be replaced with democracies, preferably by peaceful means - but they don't rule out the use of force.
The first phase in this process was the toppling of the Saddam Hussein regime in Iraq. But - as Judith Yaphe of Washington's National Defense University explains - the neo-con agenda extends well beyond Baghdad:
"I think that Iran, yes, is clearly the target that many of the neo-cons have in mind. First we're going to do Iraq - and then there's Iran, and there's Syria.
"But on that list also is Saudi Arabia, the Gulf states and ultimately even Egypt. All of the map of the Middle East was to be re-made in the shape of what Iraq would be - ie democratic," she says.
"Well, Iraq isn't going to be what they would like it to be, at least for quite a long time. Does that change their perspective? I don't think they always have a strong sense of reality," she says.
The debate between the realists and the neo-cons is often fierce, and it's unresolved.
The result is that the administration speaks with different voices.
The neo-cons are pleased when President Bush speaks out - as he has repeatedly - in support of Iranian students demonstrating for greater freedom.
In contrast, the realists applauded when Secretary of State Colin Powell recently said the factional fight in Iran - between reformists and conservatives - was a family quarrel in which America should not intervene.
Bush's realist and hardline advisers are fighting for his ear
George Perkovich of the liberal think-tank, the Carnegie Endowment for International Peace, believes there is a fundamental contradiction in policy-making.
"There are people who say, 'Let's try to make an arrangement, get a deal with the Iranian Government to stop its nuclear programme, give up these facilities that concern us - and we'll probably have to reassure them in various ways in order to get that arrangement'," he explains.
"There's another group in the administration which says, 'No, we don't deal with satanic regimes. The government of Iran is evil - we don't deal with them. We don't negotiate with them, we don't offer them anything. We want them to leave.'
"And so that conflict - between dealing with these people and saying 'No, they should just leave, there should be regime change' - makes US policy incoherent," he says.
One of the issues on which the two groups fail to see eye to eye is the question of how durable the current Iranian regime is - and what the alternatives to it might be.
Read more from Roger Hardy in this series on Iran and the US: