Closure of direct communications channels is a classic indicator of general war. If North Korea recalls diplomats, closes the borders and airport and discloses more actions indicating it is in a semi-war state of readiness, the Allies must prepare for a North Korean military incident, if not an attack by fire.
This is an enormous gamble, which brings into focus the defensive imperative. The people must be ready for war in the event the gamble fails. Kim and his advisors exude confidence but they cannot know for sure how the US will react. The absence of an Armistice also means the US and the South can shoot at will.