Leading article: A regime in trouble

Relations between Britain and Iran, rarely cordial in recent years, suffered a sharp deterioration yesterday with tit-for-tat diplomatic expulsions. Iran expelled two British diplomats, accusing them of spying, and Britain – as David Miliband announced in the Commons – expelled two Iranians in response, while insisting, as is a set piece of such occasions, that the British had done nothing wrong.

That tensions were rising on the diplomatic front had been apparent for days. The families of British diplomats were evacuated yesterday. What is more, Iran's accusations conformed to a well-worn rule: the more beleaguered a regime feels, the more it looks for a scapegoat beyond its borders. And where Iran is concerned, the role of scapegoat – for historical and cultural reasons – disproportionately falls on Britain. The day before ordering the diplomats' expulsions, Iran had given the BBC correspondent in Tehran 24 hours to leave the country.

BBC coverage of the Iranian election and subsequent protests has incurred the Iranian government's displeasure. This is not because it was less than scrupulously accurate, but because, via the internet and mobile phones, it was able to draw on unofficial Iranian sources. The Corporation's recently started Farsi television service has also been seen as provocative interference in Iran's internal affairs.

Now, with the ayatollahs trying desperately to hang on to power and Tehran teetering on the brink of civil unrest, is not the time to expect a new détente. Nor can it be excluded that the mood will get worse before it gets better. The UN Secretary General was another who drew Tehran's ire yesterday.

What is notable, however, is that Iran has so far fought shy of escalating hostility with the United States. It is possible that the British expulsions were intended as a warning, with Britain – seen as Washington's closest ally – cast as proxy. But so long as Tehran resists picking a new quarrel with the United States, President Obama's efforts to foster a new relationship with Tehran are not lost. If, as it appears, the ayatollahs are hesitant to burn their bridges with this US administration, that offers a sliver of hope.