YES I love our WWII discussions, they always happen every 3 years, lol.
In the hypothetical scenario that America was dealt a "decisive" blow to our military-bureaucratic command and control centers, then contingency plans at the local, state, and national levels would provide direction for the future (and if you've ever read any, some of them are a little scary). The military-bureaucratic complex of the US is so interwoven that eventually somebody or an oligarchy would take control. Plenty of movies have explored this topic.
You also have to take into account our allies. If America was disabled by some decisive attack, then the entire world would erupt against whoever made that attack. This is precisely why we may never have WWIII because mutually ensured destruction AND total war is a powerful deterrent, and exactly why wars of the future will be regional conflicts at best over negotiable national interests (which always come down to water, land, or oil -- all of which have a price).