Would you buy an international bomb-plot from a used-car salesman? Barack Obama would. Last week, Obama's attorney-general, Eric Holder, announced that the US Drug Enforcement Agency (DEA) had uncovered a plot by elements of the Iranian government to assassinate the Saudi ambassador to the US, Adel al-Jubeir.

The killing was to be carried out in a Washington restaurant by a Mexican drugs gang, Los Zetas, in return for $1.5m and a shipment of opium to be organised by the Quds Force group within the Iranian Revolutionary Guard Corps.

The story is that an informant within Los Zetas had told the DEA that he'd been approached by an Iranian-American friend of his aunt, Mansour Arbabsiar, to arrange the hire of the drugs gang to make the hit.

Arbabsiar, who ran a used car lot in Corpus Christie, Texas and had a conviction for cheque fraud, is said to have explained that, as far as his Iranian principals were concerned, it would be "no big deal" if dozens of other diners - possibly including US senators - were killed in the attack.

It is self-evident that the killing of even one US senator on American soil by Iranian proxies would lead to instant military retaliation on Iran and a catastrophic escalation of violence across the region. Why would any element of the Iranian state welcome this prospect?

And, if they did, why risk compromising the operation by involving a cut-throat drugs gang with no allegiance to any cause in the Middle East, or political perspective of any kind?

Even if the plot were for real, would the Iranians really make a down payment to Los Zetas by wiring $100,000 directly from a known Quds Force bank account to the car salesman in Texas?

Most observers, including some on the Right, have treated the yarn with scornful derision. But this merely raises the question: why summon a Press conference to put the credibility of the attorney-general's office behind such a risible story?

One possibility not considered by lovers of conspiracy, from whatever point on the political spectrum, is that the Obama administration includes, at a high level, a number of gullible fools.

The current director of the Central Intelligence Agency is General David Petraeus. A year ago, as Nato's top man in Afghanistan, he put his credibility behind a story that preliminary negotiations on a resolution of the conflict were under way between the Karzai government with its Western backers and a faction of the Taliban intent on a peace deal.

Petraeus vouched for Mullah Akhtar Muhammad Mansour, allegedly the senior Taliban negotiator.

Initial suspicions about Mansour's credentials had been resolved when, according to the New York Times, 'Several steps were taken to establish the man's real identity; after the first meeting, photos of him were shown to Taliban detainees, who were believed to know Mr Mansour.'

When the detainees, or some of them anyway, said, "Yeah, that's him all right", hundreds of thousands of dollars were handed over to Mr Mansour, who then scarpered with his takings, never to be seen again - or at least not by any of General Petraeus's trusted emissaries. It has since been suggested that Mansour was, in fact, a greengrocer from Peshawar on the Pakistan border.

The FBI has pointed out to sceptics of the Iran bomb-plot that Arbabsiar correctly identified a known Quds Force officer from an array of photographs put before him. Another possibility is that there is real conspiracy afoot, but that its purpose is not to assassinate the Saudi ambassador, but to prepare the way for a US military assault on Iran - rather in the way that Saddam's alleged arsenal of WMD smoothed the way to the 2003 invasion of Iraq.

Calls on Obama to make war on Iran have been increasingly openly voiced by some on the neo-con Right in the US and by supporters of Israel. They have been joined in recent months by the Saudi and Bahraini dictatorships stridently asserting that pro-democracy protests and demands for equality for Shia minorities in their countries are being orchestrated by Iran and must, therefore, be suppressed in the interests of curbing Tehran's Islamist ambitions in the region.

On this reading, the world is being walked towards another war by the folks who in the past decade have given us Afghanistan and Iraq.

You might think they'd come up with a more plausible story. But they might think they've gotten away with it twice already, so why not go for three-in-a-row?