I wrote a series of books a few years ago that involves such a scenario. The gist of the chapter was the U.S. activated a cell in a foreign nation that had been there for years planning how to create total chaos if they were called upon to do so at the onset of a conflict. They've since been removed from active publishing.
I tell ya, it would not take much. Chicago is already on its knees just waiting to be finished off. The glue loosely holding the fabric of our society together is that most people have decently comfortable lives. Remove that comfort and the cookie crumbles quickly. It wouldn't take unit guys to do it either, though that certainly wouldn't hurt. Just a couple of angry people with nothing to lose and a little bit of creativity.