It seems almost incredible after the catastrophe of the Iraq war, but the signs are growing that the Bush administration wants to do it all over again - this time to Iran. Just as in the runup to the invasion of Iraq, the Washington air is thick with unsubstantiated claims about weapons of mass destruction; demonisation of the country's president has reached bizarre proportions; intelligence leaks about links with al-Qaida and attacks on US and British targets are now routine; demands for war from the administration's neoconservative outriders are becoming increasingly strident; the pronouncements of George Bush and his vice-president, Dick Cheney, are turning ever more belligerent - and administration sources claim that the British government is privately ready to play ball.

You might imagine after invading and occupying Afghanistan and Iraq at such huge human and strategic cost, an attack on another Muslim country would be the last thing on the US president's mind. But the drumbeat of war has been unmistakable since the summer, when Bush declared he had "authorised our military commanders in Iraq to confront Tehran's murderous activities", and the administration let it be known that it was preparing to brand Iran's Revolutionary Guards a "terrorist organisation".

Last month Bernard Kouchner, the hawkish new French foreign minister, insisted that "we must expect the worst" and "the worst is war" - while Mohamed ElBaradei, the UN's chief weapons inspector in charge of overseeing Iran's nuclear programme, warned against the "neo-crazies" pushing for an attack after 700,000 had died in Iraq on "suspicion that a country has nuclear weapons". Meanwhile, Israel's recent air raid on Iran's ally Syria has been widely interpreted as, at least in part, a power play aimed at Tehran.

This week John Bolton, the former US ambassador to the UN, used the Tory conference to call for an attack on Iran, as leaks to the US press about war preparations continued. Newsweek reported that Cheney had been discussing the possibility of encouraging Israel to launch missile strikes at an Iranian nuclear site in order to provoke Iran into "lashing out", and open the way to a wider US assault. And in the New Yorker magazine, the investigative writer Seymour Hersh reported that in a videoconference this summer Bush told the US ambassador to Iraq, Ryan Crocker, that he was thinking of attacking targets in Iran, and the British "were on board".

A Downing Street spokesman said yesterday that the "prime minister and president have never had a discussion about an attack on Iran in Iran" and that the government was pursuing a diplomatic solution. "Of course, it's the job of a lot of people to see that contingency planning is done," he added, but denied that any go-ahead had been given. The echoes of similar denials in the runup to the Iraq war, however, cannot be missed. Nor should the reference to an attack on Iran "in Iran". Both the US and British military now regard themselves as already involved in a proxy war with Iran in Iraq, as General Petraeus recently told the US congress.

What is becoming clearer is that the likely pretext for aggression against Iran has shifted from the possibility that Tehran might develop nuclear weapons to its role in supporting and allegedly arming the resistance in neighbouring Iraq and Afghanistan. The administration is increasingly convinced that it will be far easier to convince the American public of the case for war on Iran if it's seen as being about the protection of US troops rather than nuclear scaremongering from the people who brought you Saddam Hussein's WMD. So the focus of the military plans has changed accordingly: from a wide-ranging bombing assault on Iran's known and suspected nuclear sites to "surgical" strikes on the Revolutionary Guards, who the US claims are backing armed attacks on its occupation forces.

In reality, the growing confrontation between Washington and Iran has less to do with nuclear weapons or Iraqi resistance and more with the fact that Iran has emerged as the main strategic beneficiary of the invasions of Iraq and Afghanistan. Iran and its allies now offer the only effective challenge to US domination of the Middle East and its resources. It's hardly surprising that the US is alarmed by the increased influence of an avowedly anti-imperialist state sitting astride a sea of oil, now making common cause with other radical, independent regimes in Latin America. But it is of course the direct result of Bush's own policies, which have also provided an object demonstration of the advantages of possessing nuclear weapons - even if there is as yet no evidence that Iran actually intends to acquire them.

Of the three states Bush originally damned as the axis of evil, one - Iraq - had no nuclear weapons and was duly destroyed. The second, North Korea, managed to acquire some nuclear capability and is this week reaping the benefits in aid and negotiation. The third is Iran, a country surrounded by US troops and caught between two nuclear-armed US allies: Pakistan and Israel. And despite the populist Mahmoud Ahmadinejad's ugly remarks about the Holocaust, it is the nuclear states America and Israel that now threaten and have the capacity to attack Iran, not the other way round.

What should not be in doubt is that the consequences of an attack on Iran would be devastating, both in the region and beyond. Iran has the reach to deliver an unconventional armed response in Iraq, Afghanistan, Lebanon, Israel, Saudi Arabia and the Gulf - as well as on the streets of London. The economic impact could be even greater, given Iran's grip on the 20% of global oil supplies that are shipped through the Strait of Hormuz. It would also certainly set back the cause of progressive change in Iran.

Iranian leaders have dismissed the threat of attack as "psychological warfare", and no doubt the US would prefer to bring Iran to heel through political upheaval in Tehran rather than by force. But current destabilisation efforts seem unlikely to succeed, and so, short of a sudden US embrace of genuine negotiation, the chances of war before Bush leaves office look high. The likelihood of a Brown government directly participating in an attack must be small after the debacle of Iraq. But the possibility that logistical or political support might be offered is more serious. The need to step up public pressure to make sure that does not happen could not be clearer.

s.milne@theguardian.com