So, Canada has decided that it wants no part of the United States' missile defense program.

So, let's say that North Korea's Kim Jong Il goes nuts and decides to launch one of his nukes at the United States. Let's say that they miss -- NORAD determines that the nuke will fall north of the border.

Does the United States try to shoot the thing down? Or do we just have to help with the resulting environmental and human disaster that follows?