Forcible "liberation" is contrary to the principle of self determination  The absence of a civil society in North Korea makes it very difficult to know if there is a great upwelling of dissent in the country but there is certainly very little in the way of evidence of it. For the same reason, there is no obvious government in waiting, there is nothing that could take over from the triad of party, army and state that currently runs the country except and imported elite from the South.  As a result an uninvited military intervention the people of North Korea would end up, in effect being ruled by a ruling elite that they don’t know.  The influence of the regime is everywhere in the North and an occupying force would need to attempt a process similar to the disastrous de-Ba’athification actions in Iraq. The results do not seem likely to be any different.  Replacing a heavily armed rogue state with a similarly heavily armed failed state would not seem to represent much in the way of progress.  A rogue state can, at least, be mostly constrained by China, a border region that is part of a united Korea in name only offers no such opportunities for persuasion and coercion. Instead it would represent a ‘badlands’ causing difficulties for all around it on a daily basis with rampant crime taking the place of economic activity.