Donald Trump isn’t the only one to have prodded North Korea’s baby-faced dictator, Kim Jong-un. President Trump did it with Twitter – ‘Little Rocket Man’; under President Obama there were big military exercises and red-team-versus-blue-team war games at the Pentagon that led to alarming headlines about how North Korean artillery could flatten Seoul in 30 minutes. One scenario for a conflict was that America would carry out a massive, early strike to decapitate the leadership in Pyongyang.

The US might have done the same to Moscow if the Cold War had turned hot. With all we now know about the Soviet Union, could such a strike have worked? When the archives opened up, and the officers who had commanded the rocket silos began to talk, it emerged that Moscow had a Dead Hand strategy. If the leadership was gone, Soviet missiles would be launched automatically, without any order from the centre. In fact, if the leadership was no longer there to give an order to stop, there was no way to prevent a launch – and Armageddon.

‘It is virtually certain that this same secret delegation exists in every nuclear state,’ Daniel Ellsberg says in a new book, The Doomsday Machine. Yes, he’s that Daniel Ellsberg, who leaked the US government’s secret history of the Vietnam war, the Pentagon Papers, precipitating the events that led to Watergate. Before that, Ellsberg drafted the Kennedy administration’s plan for nuclear war. He writes now that ‘frequent’ leaks in the American media of US contingency plans to decapitate North Korea’s leadership have ‘very probably had the effect in that country of creating a Soviet-like Dead Hand system for assuring retaliation to such an attack.’

From Kim’s point of view, this would make sense. He’s facing a vastly superior enemy, publicly contemplating his demise. When North Korea’s foreign minister called Trump a ‘mentally deranged person’, Trump tweeted that if the minster was echoing the thoughts of ‘Little Rocket Man, they won’t be around much longer’. Trump also told the UN that if the US or its allies were attacked ‘we will have no choice but to totally destroy North Korea.’ In response, North Korea supposedly has a few nuclear warheads, and rockets to launch them. But perhaps far more importantly, they may have the poor man’s WMD: biological weapons

A report from Harvard University’s Belfer Center last October assessed that North Korea had 13 types of agents for biological weapons. These included smallpox – thought to be the most likely to be used in a weapon – but also anthrax, botulism, cholera, typhoid, Korean hemorrhagic fever, and the black plague. The South Korean Defence Ministry says the North could weaponise these agents ‘within ten days’. Pyongyang has denied possessing such internationally outlawed weapons but the US military takes the threat seriously enough to have inoculated its troops in the South against smallpox.

In one way, we’ve been here before, with the Downing Street Iraq dossier and its claim that Saddam could launch chemical weapons in 45 minutes…‘Brits 45 mins FROM DOOM’, the Sun headline screamed. A series of ifs – what if Saddam has WMD; what if he gives them into Al-Qa’eda – led the US and Britain into a pre-emptive war. Now, though, the what-ifs might lead in a different direction, towards restraint and a diplomatic solution. If President Trump doesn’t sound restrained, this might not, this time, be a case of Trump being Trump: he may be trying to make sure deterrence works. Nixon called it the Mad Man theory: your enemies have to think you’re crazy enough to push the button.

A Washington figure who has been on the inside of the biological weapons issue for decades told me he had been thinking about how North Korea might carry out an attack. His nightmare was that a few vials of smallpox virus would be left open on the New York subway: perhaps they had already come over in the diplomatic pouch. If that sounds hysterical, the US authorities have been preparing for years for just this kind of attack — whether by a state or by a terrorist group – stockpiling 300 million doses of smallpox vaccine, almost enough for every man, woman and child in the country.

Watching Trump and Kim exchange insults, it might be comforting to think that Korea is very far away, should a war start. But viruses spread. What starts in Korea or New York might not stay there, and other countries do not have the Americans’ huge stockpiles of vaccines. My friend in Washington, and others, fear a global catastrophe if war starts. So is it: Welcome back Dr Strangelove? Should we start the Doomsday Clock? We know little of how the secretive and paranoid North Korean regime thinks. We don’t know how they might react if pushed. But a regime that has survived this long is capable of rational behaviour. For Kim, now, the rational thing to do might be to have smallpox ready, to be unleashed by a dead hand.

Paul Wood is a BBC correspondent and fellow of the New America foundation