Why do you think they would send a fleet?
It would only 1 missle.
It would be launched from a single small freighter or fishing boat 200 miles out to sea from the east coast.
Then BAM, instant stone age!
Bingo !
Stone age? Seriously? It would set us back a bit, but I grew up without electricity and was alive when most of the fancy crud we have now was invented, I did not live in the stone age.
My solution if that happens to us is to do the same to other countries anyway so we are all on a level playing field for the rebuild.