Re: Kim Jong-un
Well the US hasn't really tried diplomacy with NK have they?
Incorrect. There was a deal done for food and oil aid in return for a moratorium on nuclear development done in the 90s, by Clinton. The US and South Korea provide the aid, and the North comprehensively broke the terms of the deal on nuclear.
Bush (Dubya version) had a go at putting this agreement back together. There were long 6 party talks to try and resume the same deal, but with a few more safeguards, given North Korea had broken the last deal. These eventually broke down and the DPRK continued with their nuclear program.
Late era Bush and Obama decided to tell the Norks what they wanted, but not to accept offers of talks, as what usually happened was the Norks would demand something, then escalate by sinking a ship, having a nuclear/missile test or attacking the South with artillery when they didn't get it. So they went for the increased sanctions approach instead.
The DPRK are again making peacful overtures, such as the joint Winter Olympics team. It may be that China finally deciding to apply serious pressure might be working. Or not.
But to say the US hasn't tried diplomacy is bollocks. Trump hasn't been diplomatic of course, but the US policy has been relatively consistent under both parties. They tried aid for concessions and got screwed, then they tried to re-negotiate that in hopes of a better result, then tried sanctions. Trump has added making an arse of himself on Twitter to the mix, but broadly US policy hasn't changed much in the last decade.