One of the most surprising, and depressing, aspects of the party leaders' television debate on foreign and defence policy was Gordon Brown's claim that to question whether the Trident nuclear missile system needed to be replaced was a threat to Britain's security, and to question the "special relationship" was anti-American.

Twice during the debate Brown accused Nick Clegg, of being "anti-American". Labour party sources suggested later that the charge was triggered by an article the Lib Dem leader had written for the journal of the Royal United Services Institute, RUSI. "Since the Suez crisis", wrote Clegg, "UK governments have elevated the 'special relationship' above all else, giving it priority to even when it is against our national interest to do so. The US recognises that the special relationship has had its day".

Yet that "special relationship" means more to Britain than the US, and always has done. It is inevitably lopsided. The chief benefits to Britain have been privileged access to US intelligence-gathering operations, but as we have seen recently over British involvement in the abuse of terror suspects, that has come at a heavy price. Questioning the "special relationship" is not being anti-American. Far from it.

Coupled with that attack from Brown was his suggestion that proposing any alternative to replacing Trident with a new fleet of submarines equipped with US-built long-range nuclear missiles was very dangerous. When Clegg questioned the proposal, saying it should at least be included in the post-election strategic defence review, Brown responded: "I have to deal with these decisions every day, I say to you, Nick, get real, get real". It was as if the prime minister's finger was poised daily over the nuclear trigger.

The UK's Trident system, with warheads made at Aldermaston with US help, was designed in the cold war as a deterrent able – possibly – to penetrate Russian missile defences. Labour and the Conservatives seem to be jointly paralysed by the fear that proposing an alternative system, significantly reducing the nuclear arsenal, would be seen as weakness, pilloried by sections of the media.

Yet against which actual or potential enemy could Britain's Trident missiles be a credible deterrent? Who would Britain use them against? Prime ministers, present and prospective, cannot answer.

"The single biggest threat to US security, both short-term, medium-term and long-term, would be the possibility of a terrorist organisation obtaining a nuclear weapon," Barack Obama said earlier this month after hosting an international conference on nuclear security. Does the British government disagree? How would Trident deter terrorists?

A few days earlier, Obama signed a treaty with the Russian president Dmitry Medvedev, cutting the number of their two countries' operational nuclear warheads. At the same time, the Obama administration published a new Nuclear Posture Review which downgraded the importance of nuclear weapons in military strategy and said the US would not manufacture any new nuclear warheads.

The British government, meanwhile, sticks to a nuclear doctrine based on an assumption that our prospective enemies will be frightened by the principle of "calculated ambiguity". There are a number of alternatives to Trident, including a small arsenal of nuclear-tipped cruise missiles on conventional submarines. Nick Ritchie and Paul Ingram, two respected analysts, say in a forthcoming article in the RUSI journal, that there is no need to persevere with the policy of having a Trident submarine "continuously at sea". There are many who argue that there is no need to take a decision now anyway.

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