In the last two decades, our military has not had to deal with a traditional military opponent; we've been dealing with an insurgency and/or tribal warfare where we could buy allies or get a reputation as honest brokers by simply acting the way America always has acted by helping people improve their own lives rather than trying to be a colonial empire. The first Gulf War wasn't really a contest between traditional military forces; even before the collapse of the Iraqi military following the invasion of Kuwait, the Iraqi regular army and Republican Guard were mismanaged at best by a civilian, Saddam Hussein, who terrified his generals and didn't understand much about military strategy.
A serious shooting war with North Korea would not be a minor problem. They have one of the world's largest armies, and their training is at such a vicious level that our technological sophistication isn't necessarily as much of a problem for them as would be the case, for example, in a war with China or Russia.
Even if the North Koreans completely fell apart after a few weeks or months of fighting, we would be dealing with hundreds of thousands and quite likely millions of casualties not just in North Korea but also in Seoul, huge evacuations or refugee flights likely to exceed ten million people, massive worldwide economic disturbances (imagine the effect if Samsung, Hyundai, Kia, and similar companies were unable to fulfill orders for just a few months, let alone if they ceased to exist), and the very real possibility that China would decide to start doing very bad things by threatening to cause even more destruction to the American economy if we didn't settle things in Korea according to their liking.
The Kim dynasty in North Korea is not only evil but quite possibly insane. We cannot assume they will act rationally, or that the senior officials have enough knowledge about the outside world to even understand what kind of risks they are taking — or to care.
The only good thing about a North Korean invasion is that I'm much more confident in the power of the South Korean military — in which I have several relatives now serving — to fight back in ways that we won't, especially if they're attacked first.
The South Korean Army trains to fight on their own soil, they have a level of technological sophistication equal to any other major Western military, they have a level of training intensity superior to virtually all pro-US military forces with the exception of the Israelis and the Turks, and especially in a shooting war on their own soil, they're willing to do things that we won't do.
Let's just say nobody in the North Korean military or political leadership wants to still be living if the South Koreans get their hands on them during a shooting war that kills hundreds of thousands if not millions of people in Seoul, Inchon, and other major South Korean cities. There are reasons why the North Vietnamese and the Viet Cong feared the South Korean soldiers who fought with the Americans during the Vietnam War. It's not just the North Koreans who can be cruel and merciless.
war is heck, yes, we got it
Their military is crumbling due to a lack of proper maintenance, their soldiers are starving almost as badly as the populace, abusive treatment just means abused soldiers—not tough soldiers, and if you think Saddam’s generals were afraid of him, then you can’t even imagine how scared Kim’s generals are of his royal fatness.
It would be messy, but I don’t think it’s as dire as you believe it would be.
Let me testify for my father who survived the Hadong Massacre a week before his 18th birthday.
http://en.wikipedia.org/wiki/Hadong_Ambush"
Them little buggars can fight, and they're all true believers.
We should never send another man on ground above the 38th parallel. We must tell China that if a war starts we will nuke North Korea with 250, 100+KT weapons in the first 15 minutes. And, we will follow with another 1,000 for anyone who intervenes.