The only argument for nuclear deterrence that could be heard today in the Commons, from both New Labour and the Conservatives, was that the world’s future cannot be predicted; and that, in the absence of safe predictions over future threats, we must keep our nuclear weapons, just in case. This is, by the way, an admission that nuclear weapons, today, are not doing any deterrence. And it is, therefore, an admission that, today, nuclear weapons are not deterring North Korea, for example.
Now, not only this is a bad argument; this, actually, is no argument at all. Because it does not point to anything as the justification of nuclear deterrence. To justify nuclear deterrence, as to justify anything, we must point to something: in this case, we must point to, if not actual, at least possible threats. If there are no possible threats to point to, than there is no justification for nuclear deterrence. But possible threats are not even enough: being attacked from outer space is a possible threat, but in the absence of any reasons for thinking that this might actually happen, this possible threat does not constitute a justification for nuclear deterrence (if the government does indeed think that this is a reason to keep Trident, let us hear it).
So whoever cares to justify nuclear deterrence, must point to reasons for thinking that those threats might materialize. Unfortunately the government is not in the position to name any possible future threat that we have reason to think might materialize; because that would unsettle its international relations. The government, for example, cannot mention any reasons for thinking that, 50 years down the line, China might turn nasty. So, in short, the government is not in a position to justify nuclear deterrence. Make of it what you like: it might be that the government must have the right, in the interest of its people, not to make necessary claims that would, though, unsettle its relations with the international community. Or it might be that, in the absence of a public justification to Parliament of nuclear deterrence, the government is asking the Commons to vote on an unjustified motion – and that Parliament should never vote for an unjustified motion.
But the Conservatives, which are using the same arguments as the government, are not in that delicate position. The Conservatives can, therefore, name their reasons; they therefore ought to: let us hear them!
UPDATE: MPs voted to renew Trident, 409 to 161, majority 248. According to The Guardian, around 85 Labour MPs voted against the Government.