LONDON -- The issue of an independent nuclear deterrent has now once again become a prime topic of debate in Britain.

Of course, the question of whether Britain should retain nuclear weapons has been a subject of fierce political argument all along since the atomic bombings of Hiroshima and Nagasaki in World War II. But somehow the outcome has always been a foregone conclusion.

Britain had to keep its nuclear weapons as long as other states were threatening the world with such horrific weapons. National security, went the argument, depended on deterrence, on sending the clear message that any nuclear attack would mean nuclear retaliation and that the attacker would be inviting annihilation -- the so-called theory of mutually assured destruction.

This was the argument that always won out, not least because it seemed to work.

But suddenly there is a new context and the issues are not so clear cut. Most obviously there is no looming Soviet threat. Indeed, Britain faces no immediate nuclear threat from any other state.

Who, then, is the enemy and how can that enemy be targeted? Are multiple warheads, carried by nuclear-powered submarines, the answer to rogue states and terrorists who might have got their hands on nuclear materials? And how does Britain's deterrent, which relies heavily on missiles supplied from a common U.S. pool, contribute to the general cause of preventing further nuclear proliferation, and, if possible, promoting worldwide nuclear disarmament?

The issue has arisen in Britain for the simple reason that the present Trident nuclear system is coming up for renewal. Or to be more precise, the delivery vehicles for Trident missiles, the four enormous Vanguard-class nuclear-powered submarines, are due to wear out sometime in the next dozen years. Since it is said to take longer than that -- maybe about 14 years -- to design and build their replacements, it is claimed that a decision has to be taken now to start the replacement process.

This is coupled with the fact that while the present type of American-built Trident missiles will continue to be available for several decades, eventually there will be an upgrade and a followup missile, so the new submarines would have to be designed with that in mind.

Not only is it all very long term, but every step in this sequence is debatable and is indeed being vigorously debated.

First, is it really true that decisions are needed now? Can't the life of these submarines be safely extended? Some say yes, others no. Second, does it really take all those years to build new ones? Can't other countries' designs be copied? Third, does the system really require four new submarines -- one out on patrol, one under preparation in dock, one being refitted and one spare in case of an accident or catastrophe? Would not three do just as well? And does there anyway have to be one submarine out at sea continuously? Wouldn't deterrence be just as effective if patrols were intermittent, obviously at secret and irregular intervals?

There is also the question of cost. Estimates vary but the current official estimate of some £25 billion for the acquisition and operating of a new system is probably at the low end and there are always cost overruns. Moreover, although the spending would be spread out over the years ahead, the first impact would come quite soon.

This would be at just the time when Britain's armed forces are already complaining that they are being starved of funds to meet present commitments. Not a day goes by without some army officer or spokesman declaring that resources are inadequate to fight the difficult wars in Iraq and Afghanistan, and meet dozens of other security commitments.

Above all, how can anyone be sure what the world will look a decade or so for now? How will Britain's nuclear deterrent fit into future international nuclear disarmament negotiations or into the increasingly urgent need to reform the Nonproliferation Treaty regime, which is on the verge of breaking down?

The answer is that no one knows. All that can be said is that we are in an era of complete uncertainty in which anything may happen. An "insurance policy" against unknown and unforeseen enemies is therefore prudent.

But as nuclear weapons become easier to construct, as materials and technology become easier to transfer, as nihilist extremism and suicide creeds take hold, and as the world is moving toward a big expansion of civil nuclear power, the context of the nuclear-weapons discussion is changing. In a sense the very concept of an "independent" nuclear deterrent is being overtaken by the total globalization of the issue. Any and every decision taken in relation to nuclear weapons by any country, including whether to upgrade them, becomes a part of an overall debate of massive complexity and with huge implications for the future of humankind.

What this suggests is that while the technical and cost problems of replacing the Trident weapons system in Britain may be batted back and forth, the British decision also raises much bigger issues about the role of nuclear weapons and their proliferation worldwide.

A long and intense discussion, scrutiny and analysis lies ahead and should be actively encouraged by government -- both inside Britain and throughout the international community. Immensely difficult though the eventual decision will be, any attempt to curtail deep debate, to narrow it, to head it off with questionable deadlines, or to obscure it with a smoke screen of confusing technical arguments and assertions would be extremely unwise.