I think that if they whoever they might be would go the easy route build a device big enough to take out a major city then build a few more and pop them while in an airplane over the city of their choice. Think about all the chaos the would happen if they emp'd say New York, Washington dc, and LA and then just for fun they set off a few above our ports and electrical grid control stations. Heck fry the grid in New York and LA at the same time and we'd have to call troops home to contain that mess do it in the summer and the rioting would never end cause people would riot and loot for food and water and people would die in the heat and New Yorks pretty much our financial capitol they take that out and the repercussions would be enormous.