It couldn’t have happened to a nicer secretary of state for defence. Grant Shapps was out in Florida earlier this year for the test firing of a Trident missile. The aim was for the missile to fly several thousand miles and land in the Atlantic between Brazil and West Africa. It did not go well.

What happened was that the missile did not in fact fly several thousand miles and land in the Atlantic between Brazil and West Africa, it flew several hundred yards from the sub that launched it and crashed into the sea. Plop. It did not prevent Mr Shapps being impressed though. Our missile crashing into the sea next to the submarine instead of travelling thousands of miles to its intended target, he said, “reaffirmed the effectiveness” of the deterrent. Of course it did Grant, of course it did.

Staff at the Ministry of Defence were also keen to insist we shouldn’t read too much into the missile failing the test (cost: £17m). An “anomaly” had occurred, they said – its precise nature they would not say – and in a “real-world situation”, the missile would have worked just fine. They also said there was no need to repeat the test (cost: another £17m).

But some in the Ministry of Defence said another test was exactly what was needed, for good reason. The point of the nuclear deterrent (as anyone who lived through the nuclear angst of the 80s knows in their blood) is that it’s designed to deter countries from attacking us or using their own nuclear weapons. However, one defence source pointed out that we need to be able to show that the deterrent can actually deter, and the failed test in Florida leaves that in doubt.

In addition to the weapons actually working, there’s a second crucial element to the deterrent as well: the apparent willingness of the people in charge to use them. This has led over the years to the ghastly spectacle of politicians being asked whether they would be willing to push the nuclear button. The question comes round every time there’s an election in the air and this year is going to be no exception.

Asked the dreaded question this time round was Sir Keir Starmer while he was visiting the yard in Cumbria where all the UK’s submarines are built. Like it was the first time anyone had thought of doing it, an ITV reporter said to Sir Keir “would he be willing to push the button?” and the answer was that the deterrent “only works if there is a preparedness to use it”, which sounds like a Yes to me.

Politicians saying Yes in this way always gets some people worked up. I remember Jo Swinson was asked the same thing while she was leader of the Lib Dems and when she also said yes, she was furiously condemned by CND and Nicola Sturgeon. CND said the Lib Dem leader had shown “not a moment’s hesitation about the prospect of killing millions of people”; Ms Sturgeon said it was sickening to hear the question being answered as if it was some kind of virility test; the consequences should be made clear, she said.

Read more: Mark Smith: At last, the launch of the Glen Rosa. So why am I so torn?

Read more: Mark Smith: Another one bites the dust. Wake up, Glasgow

But what kind of reaction is that? Jo Swinson, like everyone else, is aware of the consequences of using nuclear weapons; the dread of nuclear war has been deep in the British psyche since the Second World War. It’s also clear that when Jo Swinson, or Keir Starmer, says yes to the button question, they aren’t saying yes to mass destruction; they’re saying yes as part of the political and military construct that surrounds the deterrent. In that sense the nuclear button question is a pretty stupid one because there is only one possible answer: Yes.

For more background to all of this, I’d thoroughly recommend the splendid book by the submariner Richard Humphreys Under Pressure, which is an account of what it’s like to work on a nuclear sub and live with an awareness of what it can do. Humphreys says he was aware that the submarine was potentially just 15 minutes from the start of an apocalypse because 15 minutes is how long it takes between receiving the firing signal from the PM and the nuclear warheads being launched. Quarter of an hour. End of the world.

But Humphreys, and Jo Swinson, and Keir Starmer, and the interviewers asking the question, all of us basically, are aware of the difference between the theory and the reality. Humphreys and his colleagues ate, slept and worked next to the world’s most powerful weapon while also believing they would never receive the firing signal. Which of our enemies is going to take the decision to destroy their own country? The events in Israel over the weekend may demonstrate that nuclear weapons are no barrier to conventional war, but the threat of a counter-attack from the nuclear deterrent is real, therefore the possibility of a nuclear attack on us is almost non-existent.

For all of this to work however, you need a Prime Minister, or potential Prime Minister, to say yes to the nuclear question; in a sense, they have no choice. Say no – as Jeremy Corbyn would, or Nicola Sturgeon would – and the deterrent effect no longer works. We want to prevent the deaths of millions in a world where some states have nuclear weapons and show no signs of abandoning them and so a British PM says yes on the basis of a conviction that the weapons will never be used – indeed, you are saying yes to ensure they never are. That’s essentially what Keir Starmer was doing because in this mad, mad world, he has no choice.

The Labour leader also seems to get the bigger reality too that we’re not spending enough on defence, deterrent included. Labour, he said, stands by its commitment to Nato to spend 2.5 per cent of gross domestic product on defence and a future Labour government would do that as soon as resources allow.

The Herald: Trident submarine

Obviously, “as soon as resources allow” gives Sir Keir a pretty big get-out clause, but he did also say this: “My commitment to Nato and the UK’s nuclear deterrent — maintained on behalf of Nato allies — is unshakeable. Absolute. Total. The changed Labour party that I lead knows that our national security always comes first.” It’s a welcome shift away from the mad, mad world of Corbynism.

Which leaves only the tricky question of that failed missile test in Florida. Mr Shapps says the missile plopping into the sea thousands of miles from its target reaffirmed the effectiveness of the deterrent, but perhaps he should listen to the advice of that defence source I mentioned earlier. “If I was defence secretary I would insist on another test,” they said. Good point, and if and when Labour form the next government, it’s probably something they should get round to as soon as possible.