They are stupid. Most of our military assets are EMP hardened and they would only affect civilian infrastructure. Like another poster said it would take and extremely large blast from high altitude (or many spread out in unison). They are smart enough.
That said, I only hope that if that ever happened to us that our current leader, whomever that is, would have the nerve to do it to the world and level the playing field so we are not vulnerable because our leader is gutless.
If you wipe out the civilian power grid of a major city that would be disruptive enough.