North Korea continually threatens South Korea and the US with nuclear war. Its makes propaganda videos about nuking Washington D.C. It has fired artillery into South Korean territory without provocation.
If North Korea was any other country, we'd already be in the process of uniting the Koreas. Unfortunately, North Korea's closest ally is China, with one of the largest armies in the world, and as the Chinese demonstrated in the Korean War, quantity does have a quality all of its own.
However, despite the US's reservations about going to war on the PRC's front porch, the North Korean government seems bound and determined to end up in a war of some kind. Either the US and South Korea are going to attack in order to end the threat of North Korea, or China is going to intervene in order to prevent nuclear war.
The current "peace" is unstable, dangerous, and will probably not last much longer.