While there will be a nuclear attack on an American city (if America is Mystery Babylon - see Revelation chapter 18, the purpose of this announcement is not to encourage Americans to prepare. It is another psychological operation.

I believe this is the psychological effect TPTB desire:

January 6 (remember the "insurrection")+ nuclear detonation drill = possibility of "insurrectionist" patriots acquiring and using nuclear weapons.

Watch for commentaries that will further this notion.