Specialists in nuclear deterrence occupy a world that requires the coldly rational contemplation of completely insane courses of action. Under normal circumstances, this is a world that non-specialists can ignore. But, every now and then, nuclear deterrence becomes a subject of wide public concern. Now is just such a time in Britain.

The problem is that the UK needs to make decisions about the renewal of its deterrent, at a time when there is huge pressure on government spending. A semi-public row has broken out within the new coalition about whether the ￡20bn ($30.6bn, €23.7bn) cost of renewing the Trident nuclear weapons system will come out of the normal defence budget, or be paid for by the Treasury.

But this debate is far too narrow. With the number of nuclear-armed states threatening to grow, Britain probably does need to maintain a nuclear deterrent. But the weapons involved are so horrific that any semi-rational adversary is likely to be as deterred by a 1 per cent chance of nuclear annihilation as by a 100 per cent certainty. Britain's recent government white paper on nuclear weapons emphasised the need for a “credible” nuclear deterrent, such as Trident, but an all-but incredible one would be just as effective.