Western leaders talk big about Iran's nuclear ambitions being ''unacceptable''. But what they say makes little difference. The Iranians really want nuclear weapons, and the rest of the world has no credible way to stop them. We are going to see a nuclear-armed Iran whether we like it or not. The challenge now is to work out what that means, and what we should do about it.

Last week's storming of the British embassy in Tehran reminded us how hard Iran is to deal with. Meanwhile, last month's report by the UN's International Atomic Energy Agency confirmed that Iran is perfecting the design of a nuclear weapon as well as producing the enriched uranium needed to make it. Iran is well on the way.

It is not hard to see why Iran wants nuclear weapons. One reason is fear. Iran lives in a fractious region surrounded by nuclear-armed neighbours - Russia to the north, China to the north-east, Pakistan and India to the east, Israel to the west. And, of course, Tehran fears America, and perhaps believes that nuclear weapons will help it avoid the fate of Saddam Hussein's Iraq.

But besides fear there are also ambition and pride. Iranians see themselves as heirs to a great civilisation, and the Middle East's natural leader. They see nuclear weapons as both the necessary instrument and the natural prerogative of a great regional power. Why, they ask, should India and Pakistan be accepted as nuclear powers and not Iran?

It is a fair question.