News analysis: The nuclear deterrent Nukes. Do they still protect us? Can we afford them? Must we replace Trident?

Somewhere in the world, deep beneath the waves, a British nuclear submarine is on patrol. There is always one moving silently through the depths, ready to fire. If the command comes today, a hatch will open and a Trident ballistic missile will be propelled upwards, through the waves and into the sky. Covering thousands of kilometres at great speed, it will read the stars to find a position from which each of its four warheads can fall independently to their targets.

The warheads will explode without warning among men, women and children, each one with a power five times greater than the bomb that devastated Hiroshima in 1945. Hundreds of thousands of people will die immediately. The environmental devastation will be vast. Many more deaths will follow as a result, in the target nation and across the world. Meanwhile the submarine will continue its underwater patrol, the crew barely able to imagine the mayhem unleashed above the surface as fallout spreads and retaliation is ordered.

But surely nobody would ever risk such an apocalypse, would they? Britain has the Trident system - 200 nuclear warheads carried in four submarines - so that anyone who threatens this country knows they will suffer greatly in return. Trident is a deterrent. That is the theory, anyway: a theory forged in the days when two superpowers stood nose to nose, East versus West. The doctrine of mutually assured destruction was, literally, MAD.

But the world has changed. The Soviet empire was crumbling even as Trident was being planned. So who are those missiles aimed at now? Is their doomsday force any protection from the rogue states and terrorists that threaten us? Is it worth spending the estimated £25bn it will cost to replace Trident before the end of its working life in 2024?

These questions are being asked by MPs and in the Ministry of Defence after the Government said a decision on Trident would have to be made in the life of this Parliament. That's not quite true, as a new system would only take 14 years to build. But ministers have made up their minds. "As long as a potential enemy has a nuclear weapon we will retain ours," said the Secretary of State for Defence, John Reid, last week, despite a Mori poll finding 54 per cent of people opposed to a replacement for Trident.

Britain has reduced its nuclear capability to "an absolute minimum", said Dr Reid, but it faces a range of threats - from terrorist attack to nuclear strikes - that require a range of possible responses. The minister had been harangued by Labour MPs furious that they will not be allowed to vote on whether to replace Trident. "We haven't got any enemies we could possibly want to aim nuclear weapons at now," said Paul Flynn. "The case that John Reid has given is that we might possibly have the right sort of enemies in 15 years' time, which doesn't seem a good reason for spending billions of pounds."

The problem is that the four submarines that make up the Trident force are ageing. HMS Vanguard was commissioned in 1993. All the Vanguard class submarines are powered by nuclear reactors which need refuelling only every seven years. As they produce their own oxygen and fresh water, they can stay at sea indefinitely. They can circumnavigate the world under water if necessary, and their crews of 135 are routinely away from land for months at a time. But every time they dive and surface, the high-tensile steel hull flexes. Eventually there will be cracks.

HMS Victorious is at Devonport having new fuel put in its reactor and its hull modified. Vanguard has just fired its first test missile after the same work, and should be on active service soon. HMS Vigilant and HMS Vengeance are at sea or at their base, Faslane in Scotland, but the MoD doesn't like to say. Neither does it like to say exactly who, if anybody, the 16 Trident II D5 missiles on board the duty submarine are aimed at.

If any are trained on Moscow then the crew should know that the Russians have 8,500 warheads ready for launch in retaliation and another 11,000 in storage. This is more than the United States with 7,000 operational warheads and 3,000 stockpiled. China is the next biggest nuclear power, with at least 400 warheads. France has 350.

Britain has only 200 at most, which is the same as Israel. Possibly. Israel denies having any warheads. So does Iran, although its scientists are believed to be working on some. Syria may also be trying, but Libya has given up. North Korea says it has lots of warheads, so watch out - but it provides no proof.

Cold War deterrence depended on the prospect of global annihilation. Now we face the possibility of a smaller nuclear war - between India with 50 warheads, say, and Pakistan with 100 - that would have catastrophic effects on huge swathes of the planet, but not actually destroy it.

"A full-scale nuclear exchange with Russia is much less likely than it used to be," says Doug Richardson, expert in missiles for the defence analyst Jane's. The more likely threat comes from a belligerent smaller nation using nuclear force to show it cannot be pushed around. "But even if Iran, for example, were to announce that it had 10 ballistic missiles, by the time these were subdivided among its enemies it might only have one or two left to aim at the UK," says Mr Richardson. "So in response you need only a couple of sub-strategic missiles in the British fleet earmarked for Iran. It's that sort of game."

By "sub-strategic" he means a missile armed with just one smaller warhead. "Say a yield of 10 kilotons. Half the power of Hiroshima. That's all you need."

So only 70,000 people would die immediately. (Or far more than that if the same kind of bomb was sailed up the Thames hidden in a barge by terrorists sponsored by a nuclear state. Al-Qa'ida and its imitators seem unlikely to be deterred by the thought of HMS Vanguard.)

One option being considered is to develop new sub-strategic weapons and put them on aircraft or smaller, faster stealth submarines. But the trouble with making nuclear weapons smaller and more "manageable" is that the threat of mutual global destruction disappears and the deterrent stops working. The only way to revive it would be to allow every nation to develop nuclear weapons, so that none dared threaten another. That is never going to be allowed to happen.

The 186 nations that signed the Nuclear Non-Proliferation Treaty appeared to believe that those with nuclear weapons should get rid of them and those without should not develop them. But Britain and the US continue to work on new weapons while threatening countries like Iran with sanctions for doing so. The Iranians believe this is hypocritical, at least.

The International Court of Justice has ruled that the threat or use of nuclear weapons "would be generally contrary to the rules of international law applicable in armed conflict". The Campaign for Nuclear Disarmament believes that possession of the weapons makes Britain more of a target and "this money would be better spent on more useful things like education, health and housing". The nation could buy 800 schools, 60 new hospitals or hire 20,000 consultants with the £25bn it may cost to replace Trident.

The debt to America

Aldermaston is a name synonymous with British nuclear weapons and opposition to them. From the outside, the Atomic Weapons Establishment in Berkshire resembles a Sixties polytechnic with an absurd level of security. There are wooden posts around the perimeter, a fence that slopes outwards and is crowned with barbed wire, then a road along which patrol vehicles can zoom. There is a second, taller fence, topped by razor wire, that may be electrified. Beyond this are flat-roofed utilitarian office buildings, next to what look like huge grain silos and factory stacks. The site is vast, and at five o'clock in the evening the country roads are busy with cars as thousands of heavily vetted workers go home, passing armed police as they leave. They may soon have to do a lot more vetting, as new staff are recruited.

Scientists at Aldermaston are believed to have been given up to £5bn already, to spend on expertise and equipment (including one of the largest lasers in the world) that will help develop a new generation of nuclear warheads, as well as computer models to get round the ban on testing them.

The Trident warheads were designed and built at Aldermaston and its sister site AWE Burghfield, a few miles away. They are now maintained here, and will eventually be disposed of at these secretive sites in the countryside south of Reading. Vehicles loaded with warheads leave in convoy and drive up the motorways to Scotland. They used to be followed by protesters, who attempted to trail the convoy in a car from which an arrow pointed and a sign said: "Danger: nuclear bomb in transit."

Now the focus of the protest against Trident is at the Faslane peace camp. Anti-nuclear campaigners have seen support wax - with 40,000 people marching on Aldermaston in 1960, and 50,000 women holding hands around the perimeter of Greenham in 1983 - and wane, but now the issue is live again. The Government has made clear its commitment to nuclear weapons and energy. The Bomb may not be the cause of primal fear it once was, but it is definitely back.

At least decisions will be made relatively publicly this time: Trident was commissioned in secret, as a replacement for Polaris. Two other factors have also changed since 1980. The British government, influenced by America, has dropped its long-held policy of never striking first. And the myth of Britain having an independent nuclear deterrent has been exposed.

The myth was built on memories of Britain standing alone against the enemy in 1940. But that could only be repeated if the UK fell out with the USA - and if such a thing happened, America could shut down the British Trident force within 18 months, simply by refusing to co-operate.

The blueprints, engines, fuel and guidance systems are American. Lockheed-Martin, a US corporation, is one of the three companies managing Aldermaston. Washington knows where that elusive British submarine on patrol today is hiding, and where it's going. The missiles can't be fired without information from American satellites.

So MPs can rage all they like about not getting a vote, and the Prime Minister can warm his hands on Britain's apparent status as a nuclear power, but when it comes to replacing Trident, whatever the cost, one outcome is more likely than anything else: we will wait and see what America does. Then copy it.