Is North Korea getting ready for nuclear war?

Last Friday, North Korea took the rest of the world by surprise by detonating a nuclear explosion more than twice as large as the one it tested just eight months ago. Increased economic sanctions haven't worked. The speed of development and defiance of international retaliation are giving new urgency to old questions: do the US and its allies really know what Kim Jong Un is up to? Is our understanding of this country fundamentally wrong? The consensus has been that his nuclear program was more political than military — designed for internal consumption and international show. We hear some new assessments of what's at risk for the world.