The plot in which an Iranian-American from Corpus Christi, Texas, notorious locally for his Clouseau-like dimwittedness, tries to hire a Mexican gangster to assassinate the Saudi ambassador in Washington at the behest of the Iranian authorities has been greeted with incredulous hilarity across much of the world.

The allegations need to be taken seriously primarily because they show that the White House, by giving credence to them at the highest level, is seeking confrontation with Iran in the lead-up to next year's presidential election. It is shifting towards the Saudi position of seeing the hand of Iran behind its troubles in Iraq and the pro-democracy protests in Bahrain. The US is increasingly backing the Sunni side, and above all Saudi Arabia, in the struggle between Sunni and Shia which is escalating wherever the two communities live together.

The supposed conspiracy is bizarre even by the mendacious standards of stories pumped out by the Bush administration before 2003, purporting to show that Saddam Hussein was building weapons of mass destruction. Manssor Arbabsiar, an Iranian-American who has lived for 30 years in the US, had a history of serial failure in small business ventures. He had a conviction for cheque fraud and a reputation for fumbling incompetence so great that his friends suspected he might have suffered brain damage when he was knifed and beaten up for flirting with Iranian-American women. But this man, with his rather sad history of personal and business failures, is suddenly appointed a frontline operator for Quds Force, the intelligence arm of the Iranian Revolutionary Guard Corps.

He talks to a Mexican claiming links to Los Zetas Mexican drug cartel but who is, in fact, an informant for the Drug Enforcement Administration (DEA). The charges subsequently made against Mr Arbabsiar suggest that US government agents may have turned his woolly-minded but aggressive ideas into a conspiracy to bomb the US capital. The plan to blow up 150 people in a Washington restaurant appears to have originated with a DEA agent as part of its "sting" operation.

Given the expertise, deviousness and secrecy of Iranian intelligence, such a conspiracy may appear unlikely but it is difficult to prove it never happened. When it comes to motive, however, what could Iran conceivably gain by choosing this moment to provoke the US by providing it with a case for war, as Washington would certainly have if such a conspiracy existed?

What makes the Arbabsiar plot so menacing is the way the administration has highlighted it. Far from downplaying it, they decided to treat the allegations as if they were proven facts. The conspiracy was gravely announced by Eric Holder, the US Attorney General, and publicly endorsed by President Obama and Hillary Clinton, his Secretary of the State. After such a public commitment it is going to be difficult for the US to back away.

The most likely motive for the Obama administration's vigorously expressed belief in the plot is that it is preparing the ground for the 2012 presidential election. Mr Obama's economic and social policies are failing and his only undiluted successes have been the killing of Osama bin Laden in Pakistan and Anwar al-Awlaki in Yemen. By dramatising how he frustrated the fiendish plots of the Iranians, Mr Obama can present himself as the president who kept America safe, or at least protect his national security political flank from criticism by the Republicans.

Many of the mysteries of American foreign policy make perfect sense when related to the overriding need of those in power in Washington to get re-elected. In Iraq in 2003-04 and, to a lesser extent, in 2008 we journalists based there often derided US actions as foolish, not realising that they were directed not at Iraqis but at the American voter.

In 2004, in the face of real disaster on the ground, President Bush was able to persuade the US electorate that progress was being made in Iraq at a moment when everybody based there could see that the country was being torn apart by a war of extraordinary savagery.

Four years later the White House was able to persuade much of the US media that something close to a last-minute victory had been won through the "Surge". American reporters in Baghdad often knew different, but their home offices in New York decided that Iraq was a non-story. I remember one US TV news team, maintained in Baghdad at vast expense, lamenting that they had not been on air for 50 days before the presidential election.

But US electoral politics have real repercussions on the ground in the Middle East. Saudi officials were quick to say that the US was coming around to its belief that Iran was behind the disturbances in Bahrain, though nobody has produced any evidence of this. In Iraq, any attack on US forces by Shia militia groups is very publicly denounced by American generals as being organised from Iran, while there is hardly a mention of horrendous bombings of civilian targets in Shia parts of Baghdad.

Domestic American political needs become intertwined with the vicious sectarian feuds in the Muslim world. In Iraq, for instance, the American account of what happened during the Surge is that the most of the anti-occupation Sunni insurgency, usually called the Awakening movement, rose in revolt against al-Qa'ida in Mesopotamia. Violence fell and US military casualties were reduced almost to nil, but seldom mentioned is the fact that al-Qa'ida as well as the Awakening forces stopped attacking American troops, though the jihadis continued killing Shia civilians, soldiers and officials. The US commander General David Petraeus and his supporters were keen to keep quiet about the fact that al-Qa'ida in Mesopotamia had reached a tacit understanding with the US military.

The White House may want to promote confrontation with Iran at this moment for domestic political reasons, but does it actually want war? It seems unlikely, given that the US position in the Middle East is weakening because of the loss of Egypt as an ever-loyal supporter and the rise of Turkey.