What SHOULD the USA do if terrorists NUKE us?

[Silverhair]

Neither candidate has addressed this issue. They have both talked about prevention. During the Cold War, our policy on what would happen if attacked was well known - massive retaliation. There was much public discussion on the morality and practicality of such a policy. The public voted overwhelmingly for candidate who supported MAD. (Mutually Assured Destruction) There needs to be discussion now on what we will do. Even Bush himself has stated that we may not have someone to retaliate against if we get hit with a nuke.

So here is the scenario:

YOU are President. It is March 2007. You have had much success in your presidency and everything seems to be improving. But despite your best efforts, the worst still happens. We get hit with a suitcase nuke of about 2KT yield. The bomb is detonated close to the pentagon. (A blast of that size is about right for a suitcase nuke. It will wipe out the pentagon and the very close area around it, but the city itself will not be greatly harmed. Fallout will usually go out to sea.) It is now two days later. You have done the standard address to the nation and the proper disaster relief stuff. Now to numerous media outlets and on the internet AQ posts very credible proof that they are responsible. They make no demands but just end the tape with the statement, "Death to America".

What do you do?
NOTE: Please do not respond with, "Under my Presidency that would not happen." That is ducking the question. Despite your best efforts - IT HAPPENED. Now what? Emphasis of the question is on international affairs side of the issue, please.