On this day, 65 years ago, near a remote island off the Australian west coast, a bomb was exploded in the hull of the empty navy frigate HMS Plym, its blast two-thirds more powerful than the one that destroyed Hiroshima. At that moment, Britain officially became the world’s third nuclear power.

Both Winston Churchill, who authorised the test, and Clement Attlee, who initiated it, believed its true significance lay not in increasing Britain’s military capabilities, but in further deterring the threat of nuclear war between Russia and the west, and ultimately in eradicating that threat.

Since that day in October 1952, 17 general elections have been held in Britain, and – while debates have often raged about a unilateral versus multilateral approach – the principle that the British government should always be working towards global disarmament has never seemed in doubt.

Until now, that is. Theresa May’s manifesto earlier this year was only the third by a sitting government since Britain got the bomb that made no mention at all of nuclear proliferation and the importance of arms control. And – unlike the two others – she had no excuse.

The previous exceptions to the rule were in February 1974, when Ted Heath tried to reduce his snap election to the single question “Who governs Britain?”; and 1997, when a fag-end Tory administration was barely going through the motions against New Labour.

But in every other case, the prime minister and government of the day treated it as almost a moral responsibility to make clear their long-term commitment to disarmament, often alongside a statement of their short-term plans to retain and renew Britain’s nuclear arsenal.

Take Harold Macmillan in 1955, declaring: “In the face of [the bomb’s] destructive power, any group of men would have to be not only bad, but mad, to unleash a war. We must not only abolish nuclear weapons, but also reduce armies and armaments to a point where no one state can threaten the peace. I shall never despair of finding by agreement solutions which will rid the world of fear.”

Take Margaret Thatcher in 1983, who lambasted Labour’s unilateralist nuclear policy, but at the same time insisted that “every thinking man and woman wants to get rid of nuclear weapons”; and then, in 1987, set out detailed policies for the elimination or reduction of nuclear missile capabilities throughout the world, and claimed that “Britain is at the forefront of arms control negotiations”.

Take John Major in 1992 and David Cameron in 2015, both speaking out strongly in their manifestos against the dangers of nuclear proliferation, and insisting that they would work with international allies to prevent more countries obtaining the bomb.

So the question for the prime minister and her party is: “When did disarmament become such a dirty word?” Why are Jeremy Corbyn and the Labour party now derided for advocating policies that Tory governments once considered perfectly commonplace?

Theresa May cannot claim the circumstances are different.

After all, Macmillan and Thatcher led the country through the most dangerous periods of the cold war; Major had to deal with the break-up of the Soviet Union and the fears over what would happen to its nuclear weapons; and Cameron was involved in the high-stakes negotiations with Iran over its nuclear programme.

They all recognised – as did Labour prime ministers of different eras – that the greater the threat of nuclear warfare facing the world, the more urgent the need became for all countries to tackle proliferation and make real progress on disarmament.

Look at the situation today, especially this summer’s volatile and unresolved standoff between Kim Jong-un and Donald Trump, and it is clear that the threat of nuclear conflict is definitely no less severe than in previous decades, and is arguably at its highest level since the early 1980s.

For past governments of all parties in Britain, that was the cue to step up their efforts on nuclear diplomacy, and commit to progress on disarmament. But from May, Boris Johnson and Michael Fallon, we hear the opposite, and the explanation is as depressing as it is straightforward.

They know that Corbyn – along with me and others – is a long-standing proponent of disarmament. They know that this issue has caused tension in the Labour party over the past two years. And they therefore regard any discussion of nuclear weapons simply as a chance to misrepresent Labour as soft and divided on defence.

In that context, the silence of the last Tory manifesto makes total sense; any commitment to make progress on arms control would have made it impossible for May’s attack dogs to tear holes in Labour for saying the same.

So this most unprincipled of prime ministers chooses to ignore the issue of disarmament simply for short-term political gain, something no sitting government has done since that massive blast 65 years ago on HMS Plym.

Perhaps she needs to reflect on why that 1952 test was staged as it was. Churchill’s scientists and engineers could have set off the A-bomb in any number of different ways, but they deliberately chose to simulate the effects of a bomb exploding aboard a boat approaching land.

They did so because that scenario was one of the government’s greatest fears for how an atomic bomb might be deployed against Britain. In other words, their test simultaneously proved that we were a nuclear power, but also demonstrated our huge vulnerability to attack.

And that paradox is why, at least up until now, every government since Churchill’s has believed that – whatever the arguments that nuclear weapons are a necessary short-term deterrent – the only long-term and absolute guarantee of safety is to eliminate them entirely from the planet.

It should hardly be a surprise therefore for Labour’s leader to state that as his ambition, just as it was for Churchill, Macmillan, Thatcher, Major and Cameron. Indeed, the fact that Theresa May continues to attack Jeremy Corbyn for holding that principle is not just a massive departure from the standards of her postwar predecessors, but one of the many reasons she is not fit to stand in their – or his – company.

• Emily Thornberry is the shadow foreign secretary