The United Kingdom's nuclear arsenal is at fewer than 200 warheads is less than half the size of its peak during the cold war. Any one of these could nonetheless inflict enormous damage if ever used. Both for this and other reasons, the argument against their retention has gathered force in recent years: in part on the grounds that the country's nuclear weapons serve little purpose, in part that they bolster an obsolete view of Britain's world status that bears little resemblance to reality.
Against this, there has been an odd consensus across the Conservative and Labour parties that nuclear weapons remain essential. For the Conservatives it is part of that cold-war worldview (with roots in much earlier periods); for Labour it revolves around a perennial fear of being accused of a lack of patriotism (see "Britain's nuclear-weapons fix", 26 June 2006).
There has been occasional debate over the possibilities of further radical cuts in the nuclear stockpile. A column in this series in 2008, for example, suggested a two-step process towards complete nuclear disarmament (see "Two steps to zero", 17 July 2008). That kind of thinking gained little traction with the then Labour government, wary of any return to the kind of unilateralism the party had embraced in opposition in the 1980s.
Now, however, the issue has re-emerged in a rather unexpected way: not under a Labour government but under the Conservative-Liberal Democrat coalition that took office in 2010. Its manifestations include serious discussions within the military that question the basis of Britain's nuclear stance.
The specific development that revived the argument followed the British cabinet reshuffle of early September 2012, whose casualties included a Liberal Democrat minister in the defence ministry, Nick Harvey. In the wake of his departure, he revealed the existence of a review of Britain's nuclear forces whose documented options go very much further than might otherwise have been expected (see Juliette Jowit & Patrick Wintour, "The Plan for Trident: lock the warheads in cupboard", Guardian, 27 September 2012).
The options include retaining plans for a like-for-like replacement of the Trident submarine-based system, but also others such as a scaled-down system involving the abandonment of "continuous at-sea deterrence" (CASD). The latter has been a core feature of the UK's nuclear posture since the 1960s, requiring four ballistic-missile submarines of which one is available to be kept on station at all times. The choice to end CASD might make it possible to have three submarines instead of four, though the cost savings would not be great.
More interesting is the kind of thinking described as "warheads in the cupboard". In this scheme of things a small number of warheads might be available for delivery by cruise-missiles on aircraft or submarines, but these would not routinely be deployed and there would be far fewer support-costs.
In effect, Britain would act as though it was a non-nuclear power except in the event of an unexpected threat, in which case a limited nuclear force could be rapidly assembled. This line of argument resonates with the "two-steps-to-zero" idea, and in ordinary times would get nowhere. These, though, are not ordinary times.
At the root of the debate is the huge cost of replacing Trident with a new system designed to last at least thirty years. To acquire an accurate estimate of costs is very difficult, especially as these involve so much more than building a new fleet of ballistic-missile submarines. For a start, the nuclear-weapon development-centre at Aldermaston and the associated production facility at nearby Burghfield together cost around £1 billion a year - perhaps £30 billion over the lifetime of the new system.
In addition to the costs of the submarines and the missiles, many other facilities are needed to support the nuclear fleet, including provision of a protective screen of attack-submarines. The full lifetime of Trident's replacement is likely to require an amount closer to £100 billion than £50 billion, with much of it frontloaded in the coming decade.
This comes at a very bad time for the ministry of defence for four reasons:
* The disastrous wars in Afghanistan and Iraq, as well as taking a great human toll, have been hugely expensive in material terms. Many planes, helicopters, armoured-fighting vehicles and all kinds of equipment have simply worn out, leaving huge replacement costs
* There have been several procurement disasters with massive cost overruns. Some of them have resulted in the cancellation of programmes, the Nimrod MR4A maritime-reconnaissance plane (nine years late) being a multi-billion-pound example
* The armed forces are facing huge capital costs for new conventional systems, such as the Queen Elizabeth-class aircraft-carriers and their F-35 planes (see "In defence of greatness: Britain's carrier saga", 11 May 2012)
* The entire British defence budget is facing a period of sustained austerity.
In light of all this, it is easy to see the pressure that Trident replacement is under. But it has taken another factor to tip the pressure into a real threat: attitudes within all three services of the armed forces.
First, the army has never been particularly pro-nuclear - not least because if tactical nuclear weapons had been used in a Nato-Warsaw Pact conflict that went nuclear in (West) Germany, its soldiers stationed there would have been the first people killed.
Second, the Royal Air Force (RAF) continued for years to embrace the nuclear posture - with the V-bombers, and later with the WE-177 tactical nuclear free-fall bombs carried by Tornado, Jaguar and other aircraft. But the WE-177 was withdrawn years ago, leaving the Royal Navy in sole charge of Britain's Trident-based nuclear forces. The current budgetary constraints meant that the RAF is now facing the loss of frontline squadrons, so it is hardly surprising that it is no great supporter of Trident replacement.
Third, and perhaps the biggest surprise, within the Royal Navy itself there is a striking diversity of attitudes. It certainly has a nuclear lobby, but the crunch factor is that the nuclear element consumes so much of the budget that the rest of the navy suffers. This has led to an extraordinary decline in the number of destroyers and frigates, and even threats to the amphibious ships and the Royal Fleet Auxiliary.
Taken together, all these factors translate to meagre support for Trident replacement across Britain's armed forces - certainly far less than amongst Conservative or Labour politicians. In the context of deepening austerity and yet more prospective cuts in the defence ministry's budget, there is little sign of any reversal of this remarkable turn of events.
For many years, a large minority of Britain's population subscribed to one or other view (or even both): that nuclear weapons were irrelevant to Britain's security interests, or that the very possession of such powerful weapons of mass destruction was unethical and frankly uncivilised. It is only since 2010-11 that such perspectives have begun to acquire practical momentum, and from an unexpected direction (see Brian Cathcart, "Britain and the atomic bomb", 5 August 2005).
Nick Harvey's revelations indicate that against all previous expectations there is now a genuine prospect that the UK will reduce its nuclear capacity, and to a level that until recently would have been unthinkable within government circles. If the prospect has become tenable it is not because the anti-nuclear campaigners have won the day (though they may well have had an effect), but because so many within the armed services no longer accept that Britain needs to maintain its nuclear forces. That change could herald the end of Britain's status as a substantial nuclear power.