North Korea is a threat that we cannot ignore, because of how close they are to us, but there are also next to China, who would make the war bigger than it would need to be.
In the Korean War, Chinese supplies were sent to North Korea, which caused the war to be a LOT bigger than it should have been. Same thing with Vietnam, except with the Russians.
Ultimately, if China did support North Korea in a second Korean War, then we'd have to go to war with them, which would make a huge political mess, and a huge, bloody war, with potential for WW3. I personally think we should go to war with North Korea, with all of our allies, just in case of WW3.