Two main things. First, North Korea can now semi-credibly threaten to nuke the US, which makes invading them tantamount to political suicide in the US. It’s li

Two main things. First, North Korea can now semi-credibly threaten to nuke the US, which makes invading them tantamount to political suicide in the US. It’s likely their nuke would fail or miss or be intercepted, but no President wants to be the guy that got Hawaii or San Francisco nuked. This has always been the end-game for rogue states, and means Kim Jong-Un is basically home free. If we wanted to get rid of him we missed that opportunity when we let them get nukes. Second, South Korea elected a liberal president as a reaction to the previous conservative president who was involved in a giant scandal. He had a strong mandate to negotiate. Trump played a part as well, but it’s not immediately clear how much of an effect his policies had. On the other hand the first two things I mentioned are unambiguously big.