Everybody.
"Ok, a suitcase nuke has just vaporized lower Manhatten, whom do you propose to retaliate against?"
Start with France.
The Taliban ... no, no wait ... Al Queda .... Doh, no .... Iraq? Yeah IRAQ!!!!!
Whew, I knew I'd get it right sooner or later.
Marin county?
Take out the entire military complex of Iran Syria to start with. Deport,deport, deport and intern intern intern within the US.
Repeat as neccessary - Without military power chaos will ensue they will kill each other in mass.Vaccum of power.
Why nobody, of course.
The new way of war, devised by 9th century primitives, is guaranteed to be 100% "retaliatory free"
I'm beginning to endorse the concept of counter-asymmetric war, equally "retaliatory free".
Muslim capitals and major cities will begin to disappear, one by one, and nobody, but nobody will have the slightest clue how the nukes were delivered, or by whom.
(Assuming, of course, that the the same stardard of "proof" continues to be demanded.)
"...Ok, a suitcase nuke has just vaporized lower Manhatten, whom do you propose to retaliate against?"
Well, I suppose if it's only "Lower" Manhattan thatmeans it hasn't reached 45th and 1st Ave., right?? In that case, the U.N. sounds good, no?? : )