Cocksure Kim Jong-un is raising the nuclear stakes. Is it time for South Korea to follow suit? | Simon Tisdall

Under Donald Trump, the US may become an unreliable ally. The case is getting stronger for Seoul to build its own deterrent

So-called frozen conflicts can suddenly turn hot without warning. Look at Ukraine, Syria or Armenia-Azerbaijan. Might Korea be next? For almost three-quarters of a century, an armistice - not a peace treaty - has prevented old foes North Korea and South Korea tearing each other apart. Their respective backers, China and the US, underwrote a chilly cold war status quo.

Now, momentously, the ice is cracking. But it's not a political thaw. Mutual hostility is undiminished. It's not because Kim Jong-un's impoverished hermit kingdom is imploding, as often predicted. Rather, it's because North Korea, buoyed by new friends in high places, is on a roll while South Korea is suffering a very public meltdown. In short, things are hotting up.