This is a great thread.
As for the OP, I think a first strike would come from a rogue like NK of Iran or some such country or ideal that has had a hardon for American life and freedom in general for a very long time.
Much the same as September 11 2001, it took a little time to sort out where the attacks came from and who the initial perps were.
The response did not happen overnight.
Unless it was a multiple ICBM strike where the players who have them would be easily identified I doubt there would be a knee jerk reaction to respond.
If it was USSR or China, it would be obvious and the response would be one that neither country could recover from, they may have the nukes but they do not have the layered defenses that the USA has.
A retaliatory strike would be something that they could not prevent or survive.