Under an Iranian nuclear umbrella, terrorists will be able to act with impunity and its neighbours will enter into a dangerous arms race. Less understood are the dynamics that will emerge even if Iran chooses not to use the bomb against its enemies. It matters little that Tehran may act rationally. The possibility of an uneasy peace that a nuclear equilibrium may guarantee tells us next to nothing about the conventional proxy wars that nuclear powers wage against one another. During the Cold War, the price of equilibrium was the recognition of spheres of influence. If Iran goes nuclear, the Western world will have to negotiate a Middle East Yalta with Tehran - one that may entail a US withdrawal, an unpleasant bargain for the smaller principalities on the Gulf's shores and an unacceptable one for Israel and Lebanon's Christians.

And in the end, we may not avoid a conflict either. Even the Soviet Union and the US teetered on the brink of nuclear war during the Cuba missile crisis. It happened between two countries which knew each other well, had diplomatic relations and kept important official and discreet channels of communication open even as they competed for ideological dominance. Iran and many of its prospective nuclear adversaries do not share such luxuries - there are no Israeli or American embassies in Tehran, no hotline between the Supreme Leader and the Saudi King. The potential for misreading, misunderstanding and miscalculating is immense. We cannot afford this risk. That is why Iran must be stopped.