The thought of any country developing and testing nuclear weapons is unsettling but the North Korean regime has a history of antagonistic and unpredictable behaviour. The country's relationships with South Korea and the USA have never been good but they have recently deteriorated to a new low.
But equally unsettling is the attitude of President Trump to this aggression. Rather than calm the situation, he has further added to the rhetoric. Like Ronald Reagan in the 1980s, who claimed that the USSR was 'the focus of evil in the modern world', then joked about bombing Russia, Trump has been talking about military response rather than ways to calm things down.
In that respect, it feels like the worst parts of the Cold War again. The times in the 1960s, 1970s and 1980s when the tension heated up and it seemed one or both sides would launch weapons: the Cuban missiles crisis, the Strategic Defence Initiative (nicknamed Star Wars) and US cruise missiles in Europe. All of these upped the ante and pushed both sides towards conflict.
Obviously, there was never a point where the USA or USSR actually used nuclear weapons, but it did come close. And this was between leaders who had seen the effects of Hiroshima and Nagasaki and were well aware of the risks to either side.
The worry now is that Kim Jong un and Donald Trump are quite different politicians. They make Ronald Reagan and Yuri Andropov seem like calm pragmatists. The risk of nuclear war feels more real than it has done for decades.