In this interview with Haaretz, French Foreign Minister Bernard Kouchner makes a point that often gets lost in the chatter about the Iranian nuclear programme. He

dismisses the idea that even one nuclear bomb makes the Iranians immune to attack […] “I honestly don’t believe that it will give any immunity to Iran.”

And he later says that,

contrary to Israeli intelligence estimates that Iran will have a nuclear capability in 2009, “the assessment of our intelligence people is a bit longer - apart from two years, they say. But honestly - I think that you are well informed, and so are the Americans and so are we. It has always been the case between two and four years. But to make what? One bomb.”

Let’s imagine that in a couple of years the world wakes up to news that seismic monitoring from abroad indicates that Iran has tested a nuclear device. What would immediately change? Well, making a device go off at the bottom of a shaft dug in the desert isn’t the same thing as being able to put one on top of a ballistic missile, launch that missile at Tel Aviv  and be sure that it will explode when it gets there. It would be an important step in that direction, of course, but it wouldn’t immediately make Iran vulnerable from attack due to fear of nuclear retaliation.

In fact, far from making Iran invulnerable from attack, such a test might make it much more likely. The pretense that Iran was developing nuclear energy for peaceful purposes would be over and Israel and the United States would be presented with the option of mounting an attack to destroy Iran’s nuclear facilities before a sufficient number of usable weapons could be produced.

I can hear someone down the back shouting “But what about North Korea!”  North Korea is a different case. Its 2006 test of some kind of nuclear device of dubious military potential may best be seen as extension of its long-standing policy of pan handling for aid  backed up by dire threats, and occasionally, dire deeds. The North Korean regime, whose hellish nature makes Iran look like Sweden by comparison, is not bent on ending the USA’s hegemonic role in North East Asia or conquering South Korea; what it wants is for those nations, and others, to provide it with the means to feed its starving population. Deterrence was the only viable option against North Korea before it fired its atomic pop gun and it’s the only viable one now.

Deterrence is not the only viable policy option against Iran now and this won’t immediately change when it produces its first atomic bomb.