Diplomacy has not been tried. The last 20 years has been US threats and intimidation focused on regime change and containment.  Zero diplomacy. So diplomacy has not been tried.What has failed is threats. 20 years of threats and nonsense  has led to increasing nuclear capacity of North Korea. This really got started  when Bush attacked Iraq illegitimately and illegally. That got the North Koreans nuke program going.  They believe (and I agree) its inevitable that the US will attack North Korea. Its just a question of time.  Maybe they should. Nukes are very bad. North Korea is not a nice country. Kim is not a nice person. You know all the same lies and pretexts, just like Saddam Hussein. So lets just murder him and many North Korean people already and get it over with. 
But lets not pretend that diplomacy has been tried because it hasn't for 20 years and lets not pretend that America and Americans are the good guys or "international police". They are not.