EMP (from small nuke), like nuclear, chemical, or biological are not only inviting even the person you attack and invitation to "go Roman" in response, it isn't all that hard to do foresics on nuclear material, chemicals, and biological agents in order to find out where they came from - even if you have no idea who hit you. That makes them less likely to happen with "rational" opponents. (yes, I know - a lot of "irrational" terrorists would love the chance).
You can call CBRN "Symetrical Asymetric Warfare." We've been faced with it for awhile. What if there was something that didn't kill anyone, didn't physically destroy anything, left no trace - but in a pass moves you back to the 18th Century? Something that would be exceptionally difficult to make a proportional response to - or any response to if you can't find a "finger print" of what hit you. Something like this;
The Counter-Electronics High Power Microwave Advanced Missile Project (CHAMP), led by Boeing's Phantom works, promised to change the face of contemporary warfare, and its test was a complete success. CHAMP flew over the Utah Test and Training Range last Tuesday, discharging a burst of High Power Microwaves onto the test site and brought down the compound's entire spectrum of electronic systems, apparently without producing any other damage at all. Even the camera recording the test was shut down. Struggling to contain his enthusiasm, Boeing's Keith Coleman says, "We hit every target we wanted to. Today we made science fiction into science fact."OK. We can make it years before anyone else. We can make it smaller, more accurate and more powerful. But ... make a clunky one. Put it on a GIV. Put it on a tramp steamer. Put it in a semi. What would be the impact of such a "harmless" weapon in NYC? DC?
With our love of COTS - in the middle to the Strait of Hormuz as the CBG passes through? "Back in the day" we spent a lot of time on EMCON and electronic hardening of equipment - mostly because we also planned for nuclear war. In the late 90s though, it became a lower priority, waived in most cases. Now? You can do the math. What do you need to make such a weapon? Mostly brains and a little access to technology. Give 2nd or even 3rd level nations 20 years.