Nestled deep in George Bush’s latest $190 billion request to Congress for emergency funding for the wars in Iraq and Afghanistan is a tantalising little item that has received scant attention.

The US Department of Defence has asked for an additional $88 million to modify B2 stealth bombers so that they can carry a 30,000lb bomb called the massive ordnance penetrator (or MOP, in the disarming acronymic vernacular of the military). The MOP is an advanced form of a “bunker buster”, an air-delivered weapon with an explosive capacity to destroy targets deep underground. Explaining the request, the Administration says it is in response to an “urgent operational need from theatre commanders”. What kind of emergency could that be?

It’s possible that the US Air Force wants more firepower in the hunt for Osama bin Laden and al-Qaeda as they skulk in their caves in Afghanistan. But that wouldn’t require stealth bombers – the sleek, black-skinned, radar-dodging darts of the US military. The Americans own the skies over Afghanistan and Iraq and could, if they wished, blanket the two countries with all manner of bombardment from a few thousand feet in broad daylight.

So what lies somewhere between Iraq and Afghanistan that might demand the urgent deployment of a stealth aircraft that can quietly drop a 30,000lb bomb and destroy something several storeys below ground? The secret wine cellars in Tehran that house the illicit stash of vintage clarets belonging to the Iranian President, Mahmoud Ahmadinejad? The vast collection of grey polyester suits and Iranian goody bags that lie in wait for the next batch of luckless British sailors?

Pat yourself on the back if you correctly identified the subterranean nuclear enrichment facilities operated by the Iranian Government in its pursuit of an epoch-altering Bomb.

The debate in Washington about what to do with the increasingly recalcitrant and self-confident Iranian regime has taken a significant turn in the past few weeks. And the decision to upgrade the bombing capacity of the US military is perhaps the most powerful indication yet that the debate is reaching a climax.

A number of developments have tilted the argument towards a more assertive US posture. First, even the ever-optimistic Sisyphuses at the State Department are tiring of pushing the rock of diplomatic futility up the slopes of Russian intransigence. It’s clear even to the most starry-eyed of Russophiles that Moscow, under its breathtakingly arrogant and ambitious President, has no intention of lifting a finger to help the US and its allies with serious economic measures that might persuade the Iranians to disarm. (This is a staggeringly shortsighted decision given the threat posed by a militant, nuclear-armed Islamist state on the borders of former Soviet republics with large Muslim populations.) Meanwhile, China too continues its myopic pursuit of global commercial opportunities to the exclusion of its long-term strategic security.

At the same time other diplomatic signalling has become much more favourable. France has long been an advocate of a hardline approach towards Iran and Nicolas Sarkozy’s Government has recently indicated its willingness to put its military wherewithal where its mouth is.

Then there was the resignation last week of Ali Larijani as the chief Iranian nuclear negotiator. Those of us who have watched Mr Larijani’s deadpan performances over the years as he has explained Iran’s wholly peaceful intentions to international conferences have wondered how long he could keep it up.

It’s possible he just got fed up with the effort of telling all those lies to disbelieving Western audiences. But the more likely explanation, among Iran watchers in Washington, is that he failed to convince the religious leaders to whom he was accountable to rein in the lunatic reveries of Mr Ahmadinejad. So much, by the way, for the old comforting contention that the mullahs didn’t really share the President’s apocalyptic messianism on the issue of the Bomb.

Another significant development was what happened last month when Israeli jets attacked a target inside Syria. The details remain murky but

it looks increasingly as though Israel may have pulled off a near-repeat of its 1981 takeout of the Iraqi Osirak nuclear reactor. The word in parts of the US Government is that the assault went encouragingly well, defanging an emerging threat, and, crucially doing so without obviously provoking a devastating backlash against Israel and its allies.

Iran is not Syria, of course. Its tentacles, through terrorist networks around the world, are much more extensive. But the biggest argument within the US against military action in Iran has always been that such a move would inflame public opinion, causing the Iranian people – who despise their regime perhaps more than the Americans do – to rally around the Government, while, at the same time, not doing enough to set back the nuclear programme.

Now the US thinks it has the intelligence and the military capacity to undermine the Iranian threat seriously, and the costs of doing so may not be as high as once seemed.

Of all the silly arguments that pass as conventional wisdom in this debate is the claim that the US would be crazy to start a war with Iran. It’s a silly argument because America is already at war with Iran. Every day US soldiers in Iraq are attacked by Iranian-financed paramilitaries, with Iranian-produced weapons in pursuit of Iranian political objectives. Iran is manipulating the Iraqi Government in ways that undercut the steady progress the US is making in Iraq.

The only real question about the next phase in this war is whether an escalation by the US, in a pre-emptive strike against Iranian nuclear facilities, would further American – and Western – objectives, or impede them. The evidence is increasingly suggesting that the costs of not acting are equal to or larger than the costs of acting.

Military action is not inevitable; yesterday the US again emphasised the diplomatic option with a strengthening of economic sanctions. And it’s still possible that someone will prevail on the Iranians to ditch their menacing and destructive aims. But it is starting to look as though, with not much more than a year left in his term, President Bush has decided, as he surveys the unedifying global territory of ideological and state-backed terror, that he needs to clean house.

And a 30,000lb MOP might be just the job.

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