It is important to stop the Iranian and North Korean nuclear programmes but it is also clear that Washington will not get much help from its allies or the UN. A "comprehensive policy towards Pakistan" would be exceedingly useful but no one has found a way to achieve this. In brief, it is far easier to recognise the dangers than to point to ways and means of averting them. Meanwhile, as the most recent progress report says, the clock is ticking.

Even the most radical sceptics do not claim that a terrorist attack with nuclear weapons is impossible and that it will never happen. Nor do those who think that such an attack is more likely than not maintain that the worst-case scenario is inevitable.

There is no certainty that it can be prevented. But it is certain that such an attempt could be made more difficult. The best way to this end is to make it as clear as possible what the reaction to such an attack would be.