Corbyn has said he would never use our nukes. What kind of deterrent is that?

When pushed this morning by Today presenter Sarah Montague on whether there would be any circumstances that he would use the nuclear option, Jeremy Corbyn said: 'No.'

In other words, Britain under his premiership would no longer have a nuclear deterrent. Deterrence requires not just the capability to strike – a capability that cannot be pre-emptively neutralised or destroyed (hence the need for submarine-launched weapons) - but also the will to use it.

Or at least the question-mark in the mind of a would-be aggressor.

We’ve been here before. The Rt Hon Jim Hacker (Yes, Prime Minister) wasn’t as certain as Jeremy Corbyn that he wouldn’t use Trident, but he thought he probably wouldn’t and therefore that it was pointless. Until the Cabinet Secretary, Sir Humphrey Appleby, explained the exquisitely Byzantine logic of deterrence:

“

Sir Humphrey: With Trident we could obliterate the whole of Eastern Europe.

Jim Hacker: I don't want to obliterate the whole of Eastern Europe.

Sir Humphrey: It's a deterrent.

Jim Hacker: It's a bluff. I probably wouldn't use it.

Sir Humphrey: Yes, but they don't know that you probably wouldn't.

Jim Hacker: They probably do.

Sir Humphrey: Yes, they probably know that you probably wouldn't. But they can't certainly know.

Jim Hacker: They probably certainly know that I probably wouldn't.

Sir Humphrey: Yes, but even though they probably certainly know that you probably wouldn't, they don't certainly know that, although you probably wouldn't, there is no probability that you certainly would.

But with Jeremy Corbyn there would be no uncertainty. All that would remain would be the capability. That itself might be enough to act as some sort of question-mark in a crisis, for a prime minister can be defenestrated and replaced with someone more hawkish, upsetting the calculations of an aggressor’s game plan.

The problem, however, is more in the 'trivial round and common task' of diplomatic and military campaigning. Nuclear powers (and aspirant powers), like hedgehogs making love, conduct their cold wars carefully. Britain renouncing its deterrent would invite the Queen’s enemies to turn up the heat.

Jeremy Corbyn no doubt reveres Nye Bevan for resigning from the Attlee government when it raided National Insurance to pay for rearmament. But he has to remember what Bevan said subsequently, at the 1957 Labour Party conference, about unilateral nuclear disarmament: 'It would send a British Foreign Secretary naked into the conference-chamber.'

That’s if he were even allowed in.