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“A man you can bait with a tweet is not a man we can trust with nuclear weapons.” The words of his vanquished opponent, , are perhaps the biggest anxiety hanging over the shock election of Donald Trump to the US presidency: can he be trusted with the power to launch Armageddon?

Every US president has constant access to a nuclear launch device only he can activate. Born in the Cold War, it is designed for when Russian nuclear missiles are detected making their 30-minute flight to the US, allowing 10-15 minutes to decide to order a counter-strike which would not be countermanded. But it could be used in other scenarios. “There is nothing to prevent a launch for very little reason,” says Paul Ingram of the , an arms control think-tank in London.

Would Trump hit the button? He has said he would be “” but has refused to rule out using nukes, asking several times during the campaign, if they are never used, “”

The answer is deterrence: so fear of retaliation will deter any nuclear attack. Trump’s refusal to rule out their use is in fact close to existing US policy, but he has also suggested using them against ISIS, even though conventional weapons would have similar tactical effects.

“The very fact that one person, whoever it is, can decide to launch a nuclear strike is very worrying,” says Ingram.

Hair-trigger alert

The president’s ability to respond rapidly is to allow the launch of 450 US land-based missiles before they are destroyed in an attack – for which reason they are kept on hair-trigger alert.

One way to reduce the risk of a rash launch would be for President Obama to take US missiles off hair-trigger alert before he leaves, which would be politically delicate to reverse. Or Trump, who wants better relations with Russia, might do it himself.

The use of existing nukes is one thing; proliferation is another. Trump has said he will “” last year’s agreement with Iran to limit its uranium enrichment. Arms control experts say we are never likely to get a better deal, so Trump’s plan could see Iran resume its efforts.

And as North Korea approaches nuclear capability, Trump has suggested its neighbours might develop their own nuclear weapons. In April he said that US allies should pay more for the nuclear protection offered by the US umbrella – or defend themselves, “”.

Japan and South Korea, under threat from North Korea, are already under pressure to do that. But both are non-nuclear states in the Non-Proliferation Treaty, and have sworn never to develop their own weapons. Their abandonment of that pledge could well be the death-knell of the treaty. While it has failed to disarm the major nuclear powers, it has kept other countries, such as Egypt and Saudi Arabia, from trying to develop the bomb.

Anti-nuclear norms

Its failures in disarmament led a large majority of UN members to vote in October to start work on a new treaty that simply bans nuclear weapons for everyone. Existing nuclear nations rejected the idea (apart from North Korea, which voted for it), as, ominously, did Japan and South Korea. Non-nuclear NATO members backed it in the hope of strengthening global anti-nuclear norms.

Those may not last long in the Trump era. He has long called NATO “” and questions US commitments to Europe. The threat of losing reliable US defence could lead to military build-up in Europe, handing nuclear deterrence to the small UK and French nuclear arsenals, which would then be more likely to go ahead with expensive upgrades.

Thomas Homer-Dixon at the Balsillie School of International Affairs in Waterloo, Canada, sees a more insidious threat. He believes Trump could pick fights abroad and incite attacks on alleged enemies at home to generate a constant “emergency” to bolster his support. Russia’s Vladimir Putin, whom Trump has long admired, uses such tactics.

“The risk of a slide into war which ultimately involves nuclear weapons is very real,” he says. “Trump has an insatiable need to dominate, and he seems incapable of ignoring a slight.” The deadly nuclear winter predicted to follow even a limited nuclear exchange could one day answer the president-elect’s question: if we have these weapons, why don’t we use them?