The only reason I'll comment is because I finished my 2nd International Relations and Economics course last semester and much of it is fresh. Like others on the board, there are better places for discussions of this sort if you want real information, but I'll throw a little out there.
As mentioned [once again by our wise NSX comrads], there is a lot of history and tension in NK/SK/US. The 'why don't we attack NK instead of Iraq at least they taunt us with their weapons' is an extremely poorly thought of idea although albiet popular. If you want to get technical, we've been on NK's ass for decades. We have thousands of troops just WAITING to go in at any notice. I have friends there as we speak in the military.
You can go off and blame Bush as much as you want, but as you TRULY learn about the powers and choices alotted to him, you probably wouldn't give him nearly so much credit, good or bad. 5k+ innocent people died and billions of US dollars were purposely destroyed. Someone had to do something, did you do something better about than Bush did? Heck if I know, but I certainly didn't. I don't care who the president is, and don't care much for similar reasoning as I refered to above. I could and have got into pages of discussion of that, not going through it again. However, for the record, Bush has made many decisions I don't agree with. Then again, until you have as many elements and complications to consider as him, I don't think you have a right to whine about it so indefinitely and close-minded.
Why don't we mess with NK? Because NK can mess with us, and like a wounded animal, it's most dangerous when it has nothing to lose. Iran is still a similar situation. Their gov't was elected by older citizens and their younger, and much more numerous citizens, are on the border of revolting. That's a strong reason why we try to leave them alone as well, we have very little to gain and a lot to lose. We have almost as many targets on us as we have on other people, and even as the world's most powerful country, we are not invincible.
And to the gentleman concerning China, of all the contries in the world, China is by far the last one we would want to go to war with. In fact, it is almost unfathomable because the IMMEDIATE economical reprecussions would send us into a depression and make war time funding already on significant strain almost impossible to produce.
I wish every American, if not every worldwide citizen, could take a few college level International Relations courses every 5 years or so to stay aware/up to date, but that is rediculous unfortunately.