Why do people think an attack on the DPRK will be easy and short?

North Korea is not a pushover state like Syria or Iraq. They have, if I'm not mistaken, the fourth-largest standing military in the world. The North Korean people have been raised since the 50's to believe the US was the aggressor in the Korean War and is waiting for the chance to finish the job, so they will not (initially at least) see us as liberators. They also have very limited or no access to the outside world, so they don't understand how far they lag behind the rest of the world in terms of standard of living. If we attack, they will fight. They have been doing basically nothing but preparing for just this moment for going on seven decades now, and we will be crossing the largest ocean in the world to fight them on their own turf. In short, this will not be a "police action." This will be a real, honest-to-god war.

I'm not saying we shouldn't take action, because the DPRK is clearly a rogue state that routinely threatens us and our allies. Nor am I saying we can't, or won't, win a war with them. But it will not be easy, and it will not be quick. I don't believe there is a limited option. If we take any offensive action whatsoever, we had better be prepared to go the full monty, because they don't seem the type to meekly take it and submit.

A war with North Korea has the potential to be our costliest conflict since the last time we fought them. If we are really going to do this, then we had better be ready for that.