We have a sophisticated nuclear defense system, but it is primarily a missile defense system. Suppose a nuclear state with commercial flights to the U.S. uses a commercial airliner refitted as a bomber flying on a filed flight plan on a regular commercial route. Let’s say the destination is Washington Dulles. Coming from the east, that flight path wouldn’t have to divert much to go in the no-fly zone over the Capitol. I don’t know how fast we can scramble jets but I’m guessing 2-3 minutes might not be enough time to get in the air and shoot it down. Even if it stayed on the normal flight path it could probably wipe out a good bit of the DC metro area and still affect our seat of government.
I suppose that would violate the Geneva Convention (I am guessing that military weapons may not be disguised as civilian vehicles) but if you are going to launch a nuclear attack that is probably not your first concern.
How do we game out this kind of fringe scenario?