The weapons that will change modern warfare already exist: Three lower-yield nuclear weapons, launched from merchant vessels near the coasts of the United States and detonated in the upper atmosphere would destroy the American economy and disrupt American society for generations.
Spending another cent on any defense R&D before hardening our electronic systems is preparing to fight the last (conventional) war instead of facing the real (terrorist/rogue state) enemy.
Not exactly true ~ but they would create havoc for a while. I think such behavior would result in the resurrection of a more ancient custom called "tribute" and whatever nation got involved in that would simply be reduced to slavery for several centuries.
I know that if a bunch of idiots shut down my Internet and TV and made my car inoperable I would want to see them put to the yoke!