What is the Iranian government up to? For once the imperial coalition, overstretched in Iraq and unpopular at home, is proposing jaw, not war. The UN security council's offer was a good one: if Iran suspended its uranium enrichment programme, it would be entitled to legally guaranteed supplies of fuel for nuclear power, assistance in building a light water reactor, foreign aid, technology transfer and the beginning of the end of economic sanctions. The US seems prepared, for the first time since the revolution, to open a diplomatic office in Tehran. But in Geneva, 10 days ago, the Iranians filibustered until the negotiations ended. On Saturday President Ahmadinejad announced that Iran has now doubled the number of centrifuges it uses to enrich uranium. A fourth round of sanctions looks inevitable.

The unequivocal statements Barack Obama and Gordon Brown made in Israel last week about Iran's nuclear weapons programme cannot yet be justified. Nor can the unequivocal statements by some anti-war campaigners that Iran does not intend to build the bomb. Why would a country with such reserves of natural gas and so great a potential for solar power suffer sanctions and the threat of bombing to make fuel it could buy from other states, if it accepted the UN's terms?

Those who maintain that Iran's purposes are peaceful clutch at the National Intelligence Estimate published by the US government in November. While it judged that Iran had halted its nuclear weapons programme in 2003, it saw the country's civilian uranium programme as a means of developing "technical capabilities that could be applied to producing nuclear weapons, if a decision is made to do so". The latest report from the International Atomic Energy Agency notes that no fissile material has been diverted from Iran's stocks, but raises grave questions about some of the documents it has found, which suggest research into bomb-making (Iran says the papers are forgeries). Those of us who oppose an attack on Iran are under no obligation to accept Ahmadinejad's claims of peaceful intent.

Nor do we have to accept the fictions of our own representatives. The security council's offer to Iran claimed that resolving this enrichment issue would help to bring about a "Middle East free of weapons of mass destruction". But like every other such document, it made no mention of the principal owner of weapons in the region: Israel. According to a leaked briefing by the US Defence Intelligence Agency, Israel possesses between 60 and 80 nuclear bombs. But none of the countries demanding that Iran scraps the weapons it doesn't yet possess are demanding that Israel destroys the weapons it does possess.

This subject is the great political taboo. Neither Brown nor Obama mentioned it last week. The US intelligence agencies provide a biannual report to Congress on the weapons of mass destruction developed by foreign states, which covers Iran, North Korea, India, Pakistan and others, but not Israel. During a parliamentary debate in March the British defence minister Bob Ainsworth was asked whether he thought that Israel's nuclear weapons are "a destabilising factor" in the Middle East. "My understanding," he replied, "is that Israel does not acknowledge that it has nuclear weapons." Does Mr Ainsworth really buy this nonsense? If so, can we have a new minister? If Iran builds a bomb, it will do so for one reason: that there is already a nuclear-armed state in the Middle East, by which it feels threatened.

But we make the rules and we break them. The non-proliferation treaty (NPT) obliges the five official nuclear states, of which the UK is one, to work towards "general and complete disarmament". On Friday, the Guardian published the notes for a speech made last year by a senior civil servant, which suggested that the decision to replace the UK's nuclear missiles had already been made, in secret and without parliamentary scrutiny. Since then defence ministers have told the Commons on five occasions that the decision has not yet been made. They appear to have misled the House.

At the Geneva conference on disarmament in February, one delegate pointed out that the "chances of eliminating nuclear weapons will be enhanced immeasurably" if non-nuclear states can see "planning, commitment and action toward multilateral nuclear disarmament by nuclear weapon states" like the UK. If the nuclear states "are failing to fulfil their disarmament obligations", other nations would use this as an excuse for maintaining their weapons. Who was this firebrand? Des Browne, the secretary of state for defence. A man of the same name is failing to fulfil our disarmament obligations.

Browne claims that Britain must maintain its arsenal because of proliferation elsewhere, just as those proliferating elsewhere say that they must develop their arsenals because the official nuclear nations aren't disarming. With the exception of France, none of the other European states feels the need to deploy nukes. But the UK keeps preparing for the last war. Of course, no one is refusing to disarm; it's just that the task keeps getting pushed into the indefinite future. Opponents of British nuclear weapons maintain that a new generation of warheads would survive until 2055.

The permanent members of the UN security council draw a distinction between their "responsible" ownership of nuclear weapons and that of the aspirant powers. But over the past six years, the UK, US, France and Russia have all announced that they are prepared to use their nukes pre-emptively against a presumed threat, even from states that do not possess nuclear weapons. In some ways the current nuclear stand-off is more dangerous than the tetchy detente of the cold war.

The danger has been heightened by the US government's current offensive. Condoleezza Rice, the secretary of state, is demanding that other countries accept her plans to destroy the last remaining incentive for states to abide by the NPT. The treaty grants countries which conform to it materials for nuclear power on favourable terms. It's a flawed incentive - as the spread of civil nuclear programmes makes the proliferation of military material more likely - but an incentive nonetheless. Now Rice insists that India should have special access to US nuclear materials despite the fact that it has not signed the NPT and has illegally developed nuclear weapons.

If she is successful, this effort - and the concomitant US demand that India is recognised as an official nuclear power - will blow the NPT to kingdom come. The treaty which survived the cold war, and which remains the most important of the wilting guarantees against global annihilation, is being nuked for the sake of a few billion dollars of export orders.

Here's where it gets really depressing. The Bush administration's proposal has been supported by both John McCain and Barack Obama. The contrast between Obama's position on India and his statements on Iran could not be greater, or more destructive of the inflated hopes now vested in him.

Mahmoud Ahmadinejad's insistence that Iran enriches its own fissile material, and the guessing game he is playing with Israel, the atomic energy agency and the UN security council is irresponsible and staggeringly dangerous. But if I were in his position I might be tempted to do the same.

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