If you want a real deterrent, take a look at Trident's cost

The old jokes are the best. Britain deplores nuclear weapons, but must possess them in order to deter people who may not entirely deplore the species-killing art. We never intend to launch our missiles, but think it prudent to give the impression that we might, if pushed, one day change our minds. Just before being rendered into ash, that is.

This is supposed to make sense, this formulation that has been repeated, with variations, for 64 years. In that time it has been said, again and again, entirely falsely, that the world has been "made safe" by nuclear weapons in the hands of people who would never - but never say never - use such weapons. The endless procession of bloody wars large and small, from Korea onwards, is held to be beside the point.

Miraculously, things may be about to change. For this you can thank, in no particular order, your favourite buccaneering banker, al Qaeda and the western defence establishment (retd). Where CND marchers, Greenham campers and consistent majorities among the British electorate have failed, bust banks and fundamentalist assassins could yet succeed.

We cannot afford to replace Trident. People who once made careers from Atlanticist assumptions and defence procurement have noticed. As represented by a report from the Institute for Public Policy Research (IPPR), Lord Robertson (lately of Nato) and Lord Ashdown have said that £20bn for the submarines, plus £5bn for a brace of aircraft carriers (up by £1bn before the steel is cut), plus £10bn for their fighter aircraft, amount to savings waiting to happen. Joined by Lord Guthrie of Craigiebank, former chief of the defence staff, Sir Jeremy Greenstock, former ambassador to the United Nations, and Sir Chris Fox, former president of the Association of Chief Police Officers, the peers identify £24bn in potential defence economies. Repentance for these sinners comes with the recognition that the "big ticket" military programmes are not only unaffordable but, in any foreseeable world, utterly unnecessary.

The lay person could have told them as much. We can argue about stupid Afghan wars another time, but as the casualties increase and the equipment scandals accumulate, many will wonder why £1bn has been mislaid on a delayed carrier construction project before it has even begun. If they know anything about defence spending, the same people will realise that the £20bn Trident figure derives from a very modest ballpark.

Yet what have we been told for six long years? That terrorism is the existential threat. The claim is also open to question, of course, but the IPPR authors imply a comparison between the atrocities in Mumbai - they could have chosen London, New York or Madrid - and what Lord Ashdown calls "museum Cold War armaments". These gargantuan fetish objects, a Maginot Line for the 21st century, fail the first test: these weapons of defence do not defend us.

And they cost. Britain, as most realise, is a bit strapped. All parties are talking, some with more enthusiasm than others, of public spending cuts: deep, soon and for a long time to come. So, throw another £1bn at the carriers? Ashdown again: "We can no longer afford to maintain full-spectrum armed forces capable of operating anywhere in the globe like a mini-United States".

We never could. France and its Gaullist legacy aside, no other medium-sized European power has entertained the delusion that pretending to keep up with the American friends - excellent customers though we have been - was ever worth it. Now, echoing retired military men on both sides of the Atlantic, the IPPR's great and good apply a cost-benefit analysis to bloated programmes in a country on an economic crash diet.

The bankers have robbed us of the means, terrorism of the motive. So morality is served by odd conjunctions of circumstance. Yet if this government or the next presses on with Trident or the carriers, the fighters or main battle tanks, familiar "simplistic" arguments will apply. They would cut public spending by 10% (minimum) for the sake of obsolete weapons?

The best that defence ministers have managed since the Cold War's ambiguous conclusion is to remind us that the world is a dangerous place. Who knew? They tell us no-one can predict the next threat. They say it with a kind of relish. But they tend not to identify the clear and present danger - Iran? North Korea? Russia again? - or to explain why new Trident boats might help.

Personally, I would be happier to hear of government plans for the future. How much are we spending, to take one example, to protect the digital infrastructure against what excitable writers call cybergeddon? This is the new, cheap and effective weapon popular with the Chinese and the Russians. Disable a country - they almost did it to Estonia, as a test - by disabling its computer networks. Should HMG therefore dispatch an aircraft carrier, software permitting?

The immediate issue is employment. Scrap Trident, as Labour's Scottish politicians sometimes say, and you risk at least 10,000 jobs. Scrap the carriers, as we reported yesterday, and skilled workers on the Clyde will be hammered. Our SNP government's rhetoric might fail to cohere, in such circumstances, with its wider political calculations. The rest of us can only wonder why a Westminster administration could not find decent alternative uses for a few of the billions at stake.

We don't need to live this way, and certainly do not need to die this way. Nevertheless, the IPPR report is another straw in a rising wind.

Put crudely, the bankers have already run through the Trident billions. Actual conflicts, meanwhile, and conflict-prevention, bear no resemblance to the threats for which the grandiose weapon systems have been designed. It is time for a truce in the idiocy wars.

So, morality: where were we? According to the Stockholm International Peace Research Institute, as reported by the BBC, the world's leading 100 defence firms sold arms worth $347bn in 2007 alone. That marked a 37% increase in real terms since 2002. So why shouldn't impoverished Britain put a small dent in such actually obscene profits?