Jeremy Corbyn during the launch of Labour's manifesto earlier this week (Picture: PA).
By Professor Michael Clarke, Distinguished Fellow, RUSI
Forces News election analyst
Defence seldom features prominently on any General Election agenda.
Until all the party manifestos are published and compared, most of the defence attention centres on the personalities of the leaders.
Who do we trust to be in charge of defence?
In this respect, Jeremy Corbyn is trying to live down the comments he made in September 2015 and again in May 2017 that he would never 'press the nuclear button' – and therefore in a prime minister’s famous ‘letter of last resort’ to Trident submarine commanders, he would not authorise them to fire their nuclear missiles in any circumstances.
The Labour Party committed itself in both 2015 and 2017 to renewing the Trident nuclear system, but following these comments he was accused of 'giving away' the nuclear deterrent he was prepared to renew and 'lowering Britain’s defences' if he was not prepared to use it.
The reality, however, is far from clear-cut.
The famous letters of last resort, all four identical versions of them, are handwritten by every prime minister within days, or even hours, of arriving in 10 Downing Street.
One of them sits, very firmly sealed, in the safe on board whichever Trident submarine is leaving to go out on patrol.
It would be opened only if all other communications between the submarine and the UK had failed for a prolonged period and the captain had reason to believe that a catastrophic nuclear war had broken out and that the government in London had been obliterated in it.
So, it really would be a last resort.
And, of course, any prime minister can write a new letter at any time if they think the international situation merits a change in their previous instructions.
At the most, it might take three months before a new letter could become 'operational' as one submarine comes in from its standard 12-week patrol and another goes out to replace it.
The assumption is that however chaotic our world currently seems to be, realistically we are years, let alone months, away from the prospect of a genuine 'survival crisis' that would threaten a catastrophic nuclear exchange.
There is assumed to be plenty of time for a prime ministerial re-think about last resorts.
Then too, Jeremy Corbyn was only saying publicly what it is believed all prime ministers, with the possible exception of Mrs Thatcher, have written in their own letters.
But as the letters are all studiously destroyed with each change of government, and no former prime minister has ever revealed what they wrote, we will never know for certain.
On the other hand, Mr Corbyn did reveal in a public statement what should be part of the secrecy and mystery of deterrence.
So was he simply being honest, or honestly being simple – wittingly or not, undermining one of the key pillars of British defence policy?
That is a matter for voters to decide as they weigh up the personalities of those who would have the security of the UK in their hands.
When we can compare the main party manifestos, we might have something else to bite on.