Donald Trump has a simplistic view of the problems facing the United States (both domestic and foreign). And that view that our problems are simple to solve extends to the threat of North Korea having nuclear weapons and developing ballistic missiles that can carry those nuclear bombs.
Trump thinks all we have to do is cajole and/or threaten (with economic sanctions) China into taking care of North Korea -- and if China doesn't do that, then bombing North Korea will solve the problem. That view was also expressed by the vice-president in the last couple of days. And sadly, some politicians on both the left and right who normally have sensible views (like Bernie Sanders and John McClain) are succumbing to Trump's simplistic assessment.
The problem is that most issues are complicated, and cannot be solved with simplistic solutions. And that is true of the North Korean issue. Other countries do not see the issue the same way Americans do, and some of those countries (China and North Korea) are not prone to give in to American threats.
This does not mean the problem can't be solved -- only that the simplistic solutions of threats and military strikes are not likely to work. As past presidents have known, this is a problem requiring a diplomatic solution -- and diplomacy can be complicated and time-consuming, but it works.
These paragraphs from Mark Sumner at Daily Kos give us a glimmer of how complicated the North Korean issue is: