"So why doesn't Bush go in?"
China.
Aside from the Korean War, what's the recent history of Chinese aid to NK? I know they turned off the oil pipeline for a few days. To me, that's a huge break in attitude since they were sending infantry divisions marching towards the Yalu.
If I had to guess, China is too interested in their international trade standing to risk pariah status from supporting a weirdo leader of a fairly insignificant swath of east asian land.
I'm sure more familiar FRreepers will educate me :-)
China.
Nope. China would be perfectly happy for North Korea to go away. All China gets from North Korea are illegal aliens. North Korea is worthless as a trading partner, unneeded as a military ally, and diplomatically disastrous as a friend.
The real answer is South Korea. The South Koreans call the shots, here. Their asses are on the line in ways that ours are not, and they are scared. Any conflict means widespread death and destruction in South Korea, guaranteed. We can't wade in without their go-ahead, and that won't come until they get invaded.
A nuclear test might change that, though.