re: Let’s suppose that a nuke takes out an American city in the wake of the saber rattlings…

Of course not. I also don't think Russia would be foolish enough to do something like that.
The more likely scenario is sneaking a dirty bomb or EMP over.
Put an EMP in a weather balloon and knock circuits out across an entire area or state would be tremendously destructive. I don't think one drifting over like the Chinese balloon would work because they would scan that see the radioactivity hopefully and knock it down early.