In one of the clearest statements of US intentions towards Iran, the defence secretary, Robert Gates, talked on a visit to Iraq in December about the inherent unpredictability of war. If they had learned anything from the past six years in Iraq, he said, it was that. Mr Gates said that military strikes would only delay Iran's nuclear programme by about "two or three years" and that significant additional sanctions were the best of a series of bad options. If Mr Gates' view still holds, yesterday's reports that the US administration was fast-forwarding deployment of anti-missile defences in at least four Arab countries, and placing ships off the Iranian coast, should be seen as a preparation not for war, but for more sanctions. Unfortunately, any US military buildup in the Persian Gulf is inherently unpredictable.

Seen from inside Iran, the threatened deployment of anti-missile defences can only be bad news. The two defeated candidates, Mir Hossein Mousavi and Mehdi Karroubi, called on Saturday for a mass protest on 11 February, the anniversary of the Islamic revolution, despite the hanging of two dissidents last week and warnings from the Revolutionary Guard that the protest would be crushed. We don't know how deep the crack is inside the Iranian regime, but with every mass demonstration and subsequent act of state repression it widens slightly.

The Iranian regime explains the increasing domestic foment to itself, and the nation at large, as the work of foreign powers. It has recently turned on German diplomats, but India has been accused of meddling in Baluchistan; Britain and the US are for ever in the frame, and there is now a list of 60 "subversive" international organisations, among them the BBC, accused of conspiring against the state. What deterrence do Patriot missiles or Aegis cruisers provide for a regime yearning for an opportunity to put a definitive end to months of political unrest? It provides them with every incentive to brand legitimate political opponents as the enemy within.

Mr Obama is signalling that he is about to withdraw the hand he extended, after the offer to enrich uranium outside Iran's borders was rejected. In truth, negotiations were too narrowly focused and given too little time – months, in comparison to the years in which sanctions have operated unsuccessfully. Instead of probing the divisions that exist between pragmatists and ideologues within the ruling elite, Mr Obama may be unintentionally cementing them. He is reverting to a policy that his predecessor, George Bush, followed on Iran, and it is far from clear whether the result will be any different. China and Russia will not follow him down this path.