What happens when a hostile nation headed by an unpredictable leader acquires nuclear weapons? That's the question the world has been asking since Donald Trump and Kim Jong-un started trading puerile insults in 2017. But as historian Jonathan Hunt reminds us, world leaders faced exactly the same dilemma over 50 years ago when China, under Chairman Mao, developed its own nuclear arsenal. Those events demonstrate that the goal of nuclear non-proliferation has always been difficult to achieve.