A British and a French nuclear powered submarine collide beneath the Atlantic. Both are nuclear armed. A near catastrophe, but also a reminder that, to quote The Abyss, we tolerate "World War Three in a can" tooling round the world's oceans.

They are part of a nuclear deterrent, a second-strike capability, though those waters have been muddied by idiots, that says no state would annihilate another state that could then annihilate them in return. These submarines give a state that capability as they are hard to find and terrifyingly heavily armed, pretty much assuring the complete destruction of an aggressor. So essential is this capability to the UK's 'defence' that it is proposed the current Trident system will be replaced at a cost of £20 billion.

But how essential is this Cold War anachronism?

As far as I'm aware only China and Russia have strategic ballistic missiles in the numbers that pose a serious threat of first-strike annihilation that would make one contemplate needing a revenging deterrence of the sort that this class of submarine represents. The likelihood of nuclear war with either of these states is, at this point in time and for the forseeable future, vanishingly small and, given the UK's membership of a nuclear armed military alliance, not retaliation that the UK need participate in.

With final approval of a renewal of Trident not until 2012 perhaps the prevailing economic climate will stall spending on more of these terrible weapons of mass destruction rather than logical reasoning. Not the best result, but a better result than the situation as it stands now when the risk of serious accident is the biggest threat we face.