The claim that Iran employed a used-car salesman with a conviction for cheque fraud to hire Mexican gangsters to assassinate the Saudi ambassador in Washington goes against all that is known of Iran's highly sophisticated intelligence service.

The confident announcement of this bizarre plot by the US Attorney General Eric Holder sounds alarmingly similar to Secretary of State Colin Powell's notorious claim before the UN in 2003 that the US possessed irrefutable evidence Saddam Hussein was developing weapons of mass destruction.

The problem is that the US government has very publicly committed itself to a version of events, however unlikely, that, if true, would be a case for war against Iran. It will be difficult for the US to back away from such allegations now.

Could the accusations be true? The plot as described in court was puerile, easy to discover and unlikely to succeed. A Drug Enforcement Agency (DEA) informant in Corpus Christi, Texas, with supposed links to Los Zetas gangsters in Mexico, said he had been approached by an Iranian friend of his aunt called Mansour Arbabsiar to hire the Zetas to make attacks. A link is established with the Quds force of the Iranian Revolutionary Guards Corps (IRGC).

None of this makes sense. The IRGC is famous for making sure that responsibility for its actions can never be traced to Iran. It usually operates through proxies. Yet suddenly here it is sending $100,000 (£63,000) from a known IRGC bank account to hire assassins in Mexico. The beneficiaries from such a plot are evident. There will be those on the neo-con right and extreme supporters of Israel who have long been pressing for a war with Iran. In the Middle East, Saudi Arabia and Bahrain have been vociferously asserting that Iran is orchestrating Shia pro-democracy protests, but without finding many believers in the rest of the world. Their claims are now likely to be taken more seriously in Washington. There will be less pressure on countries like Bahrain to accommodate their Shia populations.

In Iraq, the US and Britain were always seeing Iran's hidden hand supporting their opponents, but they could never quite prove it. It was also true, to a degree never appreciated in the US, that Washington and Tehran were at one in getting rid of Saddam Hussein and installing a Shia government. There were points in common and a struggle for influence. The same has been true in Afghanistan, where Iran was delighted to see the anti-Shia Taliban overthrown in 2001.