Well, as I recall, the aftermath of WWII created the 38th parallel and with it and the Communist Chinese regime coming into power in 1949, the question of whether the Chinese have simmered down long enough to be a non-threat to any reunified Korean peninsula is a real head-scratcher. Everyone knows about the legendary patience of the Asian toward achieving their goals, but have they had enough of a taste of communist capitalism to want to avoid a repeat of October, 1950 and just let things be? I ain't so sure...
I can most definitely see Korea dumping us should reunification happen, but that would mean an increased presence in Japan for U.S. forces. I can't imagine us simply vacating that part of the world simply because the Koreans reunified.
And based on this latest saber-rattling from the Head Nork Wacko, I just can't see reunification happening, unless it happens under his terms. And that, of course, is unacceptable.