Leading article: Britain's many-layered quarrel with Iran

Mr Hague is right not to slam the door irrevocably shut on Tehran

It is always shocking to realise how rapidly relations can deteriorate, where diplomacy is concerned – or perhaps, more accurately, when diplomacy fails. The ransacking of the British embassy in Tehran earlier this week, and the looting of homes in the UK's residential compound, seemed to erupt out of nowhere. Only two days before that, Iran's parliament had voted to expel the recently arrived British ambassador – a decision said to require the agreement of the President and the Council of Guardians before it could take effect. No matter, the mob – doubtless not without encouragement – passed at once from word to deed.

Relations between Britain and Iran now hang by a thread. The last British diplomats left Tehran yesterday. Within hours of their departure, the Foreign Secretary told MPs that he had ordered Iran's London embassy closed and given its diplomats 48 hours to leave the country. This falls a fraction short of a formal break in diplomatic relations, but not much. "Disgraceful" was one of the milder epithets applied by British officials to what had happened in Tehran.

Of course, no one should have been under any illusion that relations between our two countries were cordial, or even normal, before this week. Britain was one of those singled out for what might be termed special treatment by the Islamic regime from its earliest days in power. History provides one reason: British involvement in the overthrow of Iran's democratically elected Prime Minister, Mohammad Mossadeq; the role of British oil companies; and the perception of a certain lingering colonial arrogance. Britain's alliance with the United States – the "great Satan" – supplies another.

When relations with Washington turn additionally sour – the two countries have still not re-established full diplomatic relations following the hostage crisis of 1979 – Tehran takes its frustrations out on Britain. This offers a partial explanation for the latest turn of events. The US and Britain had led calls for new sanctions against Iran, following the latest IAEA report on its progress, or not, in developing a nuclear bomb. One of Britain's actions was to outlaw dealings with Iranian banks. The response suggests that this move may have had the potential to injure a good deal more than Iran's dignity.

Yet it might be simplistic to dismiss the attacks on British interests in Tehran as nothing more than angry lashing out. There had been signs of a tentative diplomatic rapprochement since the last crisis in bilateral relations – after an Iranian national on the embassy staff was arrested and imprisoned following the Green protests of 2009. Evidence of a new mood was the arrival of a new British ambassador. It appears, however, that the prospect of improved relations may have been unwelcome, at least in some quarters. The question to be asked is whether the attacks on British interests reflect the official view in Tehran, or a more complicated dynamic in and around Iran's leadership. External enemies always have their uses.

Signs of internal tensions have multiplied since the disputed 2009 election, which left all branches of power in the country weakened. The Arab Spring will only have compounded the uncertainty. The fact that President Mahmoud Ahmadinejad offered an apology for the embassy attack could be interpreted as a formality, or as a hint of disagreement at the top.

William Hague's insistence yesterday that diplomatic relations were not being formally broken suggests a reluctance to slam the door irrevocably shut. And this is right. There is still no conclusive proof of Iran's nuclear weapons ambitions, and the pre-emptive strike favoured in some Israeli and US circles would only make a difficult situation incomparably worse. Iran's is a troubled regime operating in a troubled neighbourhood. Outsiders need to keep a cool head, and try not to escalate tensions further.