You can’t fault Hamid Karzai, the President of Afghanistan, for ambition. Yesterday at a summit with President Ahmadinejad of Iran he said that his country might be able to bridge the divide between the US and Iran.

Never mind that no other broker has managed that since the 1979 Iranian revolution. The feat would go some way to offset Afghanistan’s most significant influence on the wider world, as supplier of nine tenths of its heroin.

But although American intransigence towards Iran shows only occasional signs of softening, it is not a ridiculous proposition, but simply one unlikely to go far. Iran has clear interests in stopping the drugs trade too, and its claims to have been helpful are justified, for all the US’s dismissive scepticism. The mere fact of the summit in Kabul is a rebuff by Karzai to the US, let alone the warmth of his welcome. “Afghanistan has strong ties with Iran – we share the same religion and language,” Karzai said yesterday. The summit follows his bitter demands that the US rein back immediately on military operations that have caused significant civilian casualties, because of the anger rising in the country towards a foreign presence.

Indeed, it is one in a long list of ways in which Karzai has set out to demonstrate his independence from his Western supporters, not least replacing a governor of the opium-ridden Helmand province, whom British forces had found particularly sympathetic to their goals, with one distinctly less helpful. It is a less extreme form of the equivocation that President Musharraf of Pakistan has practised for eight years and which is now unravelling.

In the joint press conference, Ahmadinejad claimed that “for us, a strong and stable Afghanistan is the best option”, adding that “the security of Afghanistan has a primary impact on Iran because we have long borders”.

This should be uncontroversial, although the US disputes it. The US-led invasions of Afghanistan and Iraq have replaced two regimes hostile to Iran — the Taleban and Saddam Hussein — with two much more sympathetic ones. “From our point of view, this is great progress,” a senior Iranian official said after Saddam fell.

The instability that most intimately affects Iran is the drug trafficking, and it has lost more than 3,000 members of its security forces trying to stem the convoys of brand-new 4x4 vehicles tearing across southern Afghanistan.

Last week, at the US State Department’s launch of its new Plan to Control Narcotics in Afghanistan (like a recent British briefing, an attempt to preempt a forthcoming UN report that opium crops have reached record levels), senior officials were pressed on why the US refused to work with Iran on this issue when Britain was prepared to do so. John Walters, US Director of National Drug Control Policy, acknowledged that America has “other issues with Iran” that made a relationship impossible.

One issue is Iran’s nuclear ambitions, although Britain, which shares America’s concern, has not found that a barrier to dealing with Tehran on Afghanistan. A second, harder to pin down, is the US’s suspicion that Iran is supplying weapons to the Taleban, including a particularly lethal form of roadside bomb, with the aim of undermining Western efforts at stabilisation. US officials say that Iranian weapons are entering Afghanistan in such quantities that it is hard to believe that Tehran is unaware, even if it is not ordering the deliveries.

Asked about the accusations, Ahmadinejad said yesterday: “I strongly doubt that — there is no truth in it”. He has previously pointed out that Shia Iran nearly went to war with the profoundly Sunni Taleban in the late 1990s and has no interest in seeing them back.

It would be wrong for the US to deny Iran’s understandable — even legitimate — interests in its near neighbours. It is unnecessarily provocative to deny the real help it has given Afghanistan over drugs.

The predicament of the US is that, in Afghanistan and Iraq, countries where it had hoped to install a government that would see the world its way, local ties will compete with any sense of an obligation to the West.

Times Recommends

• The ghost of Robin Cook haunts Chilcot’s feast
• The truth shouldn’t need oaths
• Violence: let’s separate the men from the boys