Would the United States have the will to respond with nuclear weapons to an attack on Japan or South Korea?
Look y'all earlier this week it came out that North Korea was planning a nuclear test. Bush told the Chinese to put a cork in it. This is just a continuation of that. Putting NK on notice that if you dare put a nuke on a missile we will take out that missile and take out all of your ability to do anything that remotely resembles making a nuke bomb of any kind.
This is just one step towards shutting NK ability to do anything but play with themselves down.