There is now such a proposal on the table.
The French Government, according to a report in The Guardian on March 19, has suggested that France and Britain pool their ballistic missile-firing submarines ("boomers"), in effect merging their nuclear deterrent forces.
This would allow some savings on operational costs, since at the moment each country always has at least one "boomer" at sea.
Under the French proposal, they could just keep one submarine at sea, hidden and invulnerable in the mid-ocean depths, ready to retaliate against an attack on either Britain or France.
That would leave the other crew, safely ashore, plenty of time to contemplate the huge can of worms that this strategy would open.
The whole policy of always having one boomer at sea is a leftover from a different era anyway.
During the Cold War, when countries worried about the other side launching a nuclear Pearl Harbour, it made sense never to have all your missile-firing subs in port where they could easily be destroyed.
It's the classic logic of deterrence.
If the other side knows for certain that at least one submarine will survive, and shoot back with dozens of unstoppable nuclear warheads, then it won't attack in the first place.
That's why Britain has four "Vanguard" class boomers and France has three "Triomphant" class boats with a fourth building: so there will always be one at sea.
However, the Cold War ended almost 20 years ago.
No great power lives in fear of an attack from any other.
Neither Britain nor France is within range of any of the non-great powers that have or are alleged to want nuclear weapons, like North Korea or Iran.
Why don't they just leave the boats in harbour, maybe taking one out for a training cruise from time to time?But if Britain and France insist on maintaining these patrols, then they have to realise that one submarine cannot provide cover for both countries.
The idea was apparently discussed for the first time when President Nicolas Sarkozy visited Prime Minister Gordon Brown in London in March 2008, but it's not likely that either man really understands the theory of deterrence.
So here, for their benefit, is a potted version of the strategy.
Let's suppose that it's a British submarine out on patrol, and some evil country strikes France with nuclear weapons, eliminating all of France's boomers in port.
Does the British submarine retaliate with its own nuclear weapons, knowing that to do so means that Britain may also be attacked by nuclear weapons?If I were French, I wouldn't trust British promises about this.
And if I were running the evil country in question, I would likewise doubt that Britain would really retaliate against me on France's behalf, knowing that I might then hit British cities too.
So deterrence fails, and all that money was wasted.
Soon after Mr Sarkozy met Mr Brown in 2008, he said in a speech in Cherbourg: "Together with the United Kingdom, we have taken a major decision.
It is our assessment that there can be no situation in which the vital interests of either of our two nations could be threatened without the vital interests of the other also being threatened."
Fine words, but not true.
In practice, when Britain has to choose between its loyalty to the European Union and its instinct to go with the United States, it almost always chooses the latter option.
France has always had less faith in American judgement and in US willingness to fight a nuclear war on Europe's behalf, which is why it spent all that money over two generations to build a truly independent nuclear deterrent force.
The British nuclear deterrent force is different, since the missiles it uses have been American ever since 1962.
There is no formal US veto over the use of the missiles that are in British submarines, but those missiles are only leased by Britain and belong to a pool of missiles that also supplies American boomers.
A missile that is in a British submarine this time around could be in an American one in its next service cycle.
The French navy must be furious at Mr Sarkozy for offering, in effect, to combine their genuinely independent nuclear force with a British deterrent force that is very closely tied to the United States.
Fortunately for France, the British navy doesn't like the idea of job-sharing either, and can be counted on to resist, undermine, and ultimately kill the idea.
The astonishing thing is that it ever got out into the public domain at all.
• Gwynne Dyer is an independent London journalist.
