I remember similar posts to that one which were posted in early to mid 2003. They were the first posts to reveal to us that they were thinking that the next hit on America would need to be an attack strong enough to prevent us from responding to it.
For some reason, I always took the "necklace" to mean chains. Then, after the August blackout, there were other posts with "necklace" that made me wonder if it could mean the power grids and how they are interconnected. This meaning would not seem to apply in the post you posted, however.
I'm going to reply privately to this, so as not to reveal anything to jihadist lurkers.
As far as responding to a problem, think about this:
Several small nukes in Manhatten would create hundreds of thousands of burn casualties.
Our burn beds are numbered in the thousands for the entire country. We couldn't respond properly to the wounded from just one event. The system fails pretty badly right there.
There is also the South African/Zimbabwe meaning for necklace. Placing a gasoline filled inner tube around the neck of a victim and lighting it up on his front lawn for a little family entertainment.
I have wondered if necklace refered to surrounding areas of the US with WMD- effectively trapping people inside.
However, looking at that fractured US map- it seems maybe they ment that the areas of destruction would encircle the US.