An armistice could be broken by any violation by North Korea.  When broken, there is no need for a formal declaration of war.  The conflict resumes when the armistice is terminated.  In this day and age, if a North Korean missile is deemed to be aimed at the US or its allies, the "war" starts again and in a couple of hours North Korea will be a radioactive parking lot.  No fuss, no muss.  No chance for thousands of troops massing for attacks or artillery barrages by North Korea.  Just a lot of burnt out corpses to sweep up after the radiation cools off.  Hopefully before a nation is destroyed, some in North Korea will realize that the current supreme leader needs to be replaced.

Under Obama, our enemies had a field day and knew the US would not respond to illegal acts like invading the Ukraine, "annexing" the Crimea, or building "islands" in the South China Sea.  The new "sheriff" in town doesn't play by Obama's rules and the world is going to have to learn to behave better.