These reflect Washington's insistence, dating back to the Islamic Revolution of 1979 and that year's hostage taking at the US Embassy in Tehran, that Iran is a state which sponsors terrorism - not only against the US but also, through groups such as Hizbollah, against Israel. To no little irritation among US allies, US companies and their foreign subsidiaries are barred from trade in oil and most other products with Iran.
Such considerations are now crumbling before both real-politik and the demands of sympathy. The Tehran authorities, as stunned as anyone by what happened on September 11, have made unprecedented gestures of support and solidarity with their opposite numbers in stricken New York.
Meanwhile, "interesting possibilities" were opening up with Iran, Colin Powell, the Secretary of State, said. President George W. Bush, too, has made clear that he wants his "crusade" against terrorism to extend to non-Christian countries, among them almost certainly Iran.
"We will work with nations that one would have felt a couple of years ago it would have been impossible to work with," he has said.
No one is pretending that Iran and the nation it still calls "the Great Satan" are about to lie down together as lambs. But obvious confluences of interest abound in the present crisis.
Washington and Tehran could well make common cause in channelling military support to the Afghan opposition group, the Northern Alliance. It is struggling after the death of its former chieftain, General Ahmad Shah Masood, to hang on to the 5 to 10 per cent of Afghanistan it controls, and joint Iranian and American support would help it to recover.
Beyond that, however, collaboration has its limits. Despite a common enemy in Afghanistan, Iran remains an Islamic theocracy and rival of America in other spheres, above all in the Israeli conflict. However much President Mohammad Khatami would like to ease his country's estrangement from America, the religious leadership that runs Iran is less amenable.
Ayatollah Ali Khamenei, the country's spiritual head, said yesterday in his first reaction to the attacks: "Islam condemns the massacre of defenceless people, anywhere and by any means." But he added: "Based on the same principle, Iran condemns a possible attack on Afghanistan which could lead to another human catastrophe."
The pattern may well resemble the Gulf War, in which Iran did not in any way take part in the US-led coalition against Saddam Hussein. This time, too, Iran is unlikely to join any coalition, or permit America to overfly its territory - still less install bases. Experts caution that, just as a decade ago, the best to be hoped for is that Iran will do nothing to interfere.
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