I don't mean to diminish the threat of this sort of thing, but I believe a so-called "dirty bomb" would be pretty easy to manufacture, would not be difficult to move into a major U.S. city, and would likely be far more effective for the hysteria it causes than the actual destruction.
I think that the difference between WMD and this type of WME is hair-splitting. depending on what it’s made from, it could be an “area denial” weapon that would make the area unusable or severely devalued for a VERY long time.
It’s unlikely that “the enemy” (that is, you-know-who) has the ability/logistics to get a working fission device here. But as you mention...
It was made pretty clear in Iraq war 1 and in particular 2 that WMD would be responded to in the “severest manner” or words to that effect, which is “diplomat speak” for the Big Bang. So, I hope that this “differentiation” described here is not a weaseling out of this.
A nuclear device of any type needs to be responded to in kind once the source is found, with absolutely overwhelming and final force. If the government of the territory where it originated “didn’t know anything about it”, well, they should have, and survivors, (if any) need to be more watchful in the future (if any).
If this actually happens, the American people are going to demand to see a sheet of lumpy glass somewhere.