Leading article: How big a stick do we need to gain respect?

A handful of Labour MPs - mostly battle-hardened veterans of the left - will today get their chance to question the wisdom of spending billions of pounds on replacing the Trident nuclear deterrent system at a meeting of party backbenchers.

As in some medieval ritual, these events have a certain pre-ordained order to them. There will be moments of high drama in the middle of the proceedings - cue here for talk of a "revolt" - but the end is predictable.

Ministers will nod and even appear to listen. They will then wheel out their tried and trusted mantra, which is that "no decision has yet been taken" and that the Government will "consult widely" when the time is right. Most MPs will docilely agree that this kind of technical stuff does need to be left to the grown-ups, and that will be pretty much that.

In the meantime, as this newspaper has reported, work will continue on a decision that Tony Blair has almost certainly taken, to be presented later in the lifetime of this parliament as a fait accompli and unveiled amid a flourish of rhetoric about how vital an updated nuclear deterrent remains to our defence needs.

More's the pity, because the arguments for and against a nuclear deterrent have changed radically since the decision was made back in 1980 to replace Trident's predecessor, Polaris, with the present system.

Then, a persuasive argument ran that Trident answered the needs of "symmetrical" warfare, which pitted one big power bloc , Nato, against another of equal size and power, the Warsaw Pact. We needed a stick that was as big as theirs, in other words. And with only two evenly matched gladiators on the set, it was also tacitly understood that no one was ever actually going to use these weapons.

But today that argument is defunct. With only one superpower left - our own ally, the United States - it is a mystery why we still need to maintain, let alone update, such a phenomenally expensive blunt instrument - or who it is aimed at. A weapons systems predicated on a Cold War, with us eyeball-to-eyeball against Russia, is irrelevant to what military wonks call asymmetrical warfare - which means a world in which countries like ours face a steady stream of glancing blows of varying strengths from a host of unseen, shifting enemies.

But a new Trident wouldn't just be hopelessly impractical against al-Qa'ida. There is also a moral and ethical argument here.

It does not behove Britain to try to cow countries such as Iran out of developing nuclear weapons if we ignore our own obligations under the 1970 Nuclear Non-Proliferation Treaty, the nub of which was that those who weren't in the nuclear club should not try to join, while those who were in it should phase out their weapons. It is hard to see how spending billions on an upgraded Trident dovetails with that pledge. It doesn't, of course, which is why the Iranian media has been full of caustic articles on Britain's nuclear plans.

And beyond these arguments is a question about what it all says of Britain's ambitions in the 21st century. What is depressingly obvious about the Trident business is that it is umbilically linked to Britain's hunger for recognition as a Big Power - a seat at the Security Council and all the rest of it. We want "respect". But do we? And when it comes to Trident, we would also probably prefer to do nothing about it at all. Trident was designed to last for years, anyway. If we sat tight, we could keep what we had - and continue to put pressure on Iran - without appearing so shamelessly hypocritical. We could also spend those billions elsewhere.