Reassuringly, at the anti-nuclear demo in London on Saturday, Jeremy Corbyn was wearing the same beard he had a generation ago. His speech had changed. He described nuclear warheads as “weapons of mass destruction” whereas we used to talk about mass annihilation. Broadly, though, he still passed as that creature the anti-nuclear campaigner has always been: the throwback; the person whom voters have rejected, modernity has superseded, real life has rendered obsolete. You could say the same about Caroline Lucas; Greens have been anti-nuclear since before we discovered climate change and all we worried about was acid rain.

And yet, there were disturbing new elements to the demonstration, breaches in the image of anti-nukery as nostalgic and irrelevant. First, the presence of a stack of people who weren’t born in the 1980s, making arguments that didn’t exist at that time.

Second, the sight of the Scottish first minister, Nicola Sturgeon, a walking, talking contradiction of the maxim that Trident is electorally essential (this has now had to be amended to read: “abolishing Trident – polling booth kryptonite to the British people in perpetuity, except the Scottish, who appear to take to it pretty quickly, all things considered”).

The context that built the pro-nuclear argument no longer exists

Finally, there’s the fact that the demonstration happened at all; we gave up protesting nuclear proliferation because the norm was too well-defended. You might not feel safer with a seat at the nuclear table, but your countrymen do; so just deal with it. Concentrate on the fights you might win. Have a climate march instead. Yes, it was defeatist, but it was an untroubled defeatism – there was never any shortage of things to march against. Taking Trident off the table wasn’t the end of the world (until it was the end of the world).

Some of the new energy comes from the injection of fresh blood: people politicised by student fees, junior doctors’ contracts, or any one of the so-called austerity measures that have caused us all to think more deeply about public spending and what its priorities do to the lives we lead. There is also the fact that the marginalisation of this issue relied on a deliberate misrepresentation of the implications of opposing Trident.

This was, and continues to be, portrayed as a value of the hard-left; as such, it was part of the extreme socialism that made Labour unelectable for so long, and from which Tony Blair gallantly and generously rescued the party. In reality, of course, it is a legitimate moral and philosophical position to say that the threat to life contained within this weaponry is too great to be justified by the interests of any nation or any ideology. Trident is like the death penalty: to be in favour of these things is quite hard-right authoritarian; to be opposed is really very soft left, if not pretty centrist. The love affair in some quarters of the Labour party with nuclear arms is more about their crisis of confidence than proof that nuclear capacity is essential to the social democratic project.

Fundamental changes are driving the revival of this movement, which are far more profound than the Labour party’s new leadership, although that has doubtless brought it into focus. The context that built the pro-nuclear argument no longer exists. Nowadays we are not nation states pitted against one another, our aggression held at bay by a rational understanding of mutually assured destruction. Our enemies – at least if we are to believe the rhetoric of our own heads of state – are death cults, to whom the possibility of a nuclear attack would be not so much a deterrent as an incentive.

Trident rally is Britain's biggest anti-nuclear march in a generation Read more

We have a different understanding of the planet now, even if we can only intermittently demonstrate that in our behaviour. The cosy glow of security you get from having a nuclear capacity only works if, in the final analysis, you can conceive of an attack on another territory as their problem, not yours. As soon as you start to consider the global consequences, any hostile nuclear event – anywhere – is a tragedy for all of us. It would be as painful to bomb as to be bombed. This puts us in a hall of mirrors, watching the solidity of deterrence disintegrate.

And while no defence programme has ever been formulated in a time of limitless public spending, the current economic context exposes the expense of Trident to ridicule. Can a nation whose health service, its pride and joy, has become “unsustainable” (Jeremy Hunt’s implication, not mine) really afford to spend so much deterring an enemy it can’t even identify, whose susceptibility to such pressure is impossible to know or to guess? Does a nation really need international status as a potential bringer of Armageddon more than it needs to educate its citizens, freely, at a tertiary level?

It’s possible that there are people answering these questions with a throaty “yes”, but you don’t need to be very far to the left to answer “no”. The Trident lobbyists may get new arguments – “what about the jobs?” one hears a bit these days, as though this awesomely expensive build-up of destructive power were really just a useful labour market lever. But their old lines have stopped working.

The world changed and they didn’t. They are the dinosaurs; they are the warhorses; they are the dreamers, living with yesterday’s truths.