One atomic bomb is bad in the blast zone and for about 3 days or so around it and downwind. The rest of the world (& US) will look at the devastation on TV and think they are glad it wasn't them.
But what is the likelihood of a single bomb? If I were a meglomaniac terrorist with power and money, I would think the best way to do it would be to truck the bombs to the desired location (cheaper and more accurate than ICBMs or whatever they're called these days), they couldn't be tracked by radar, if the CIA/FBI did get word of them they wouldn't do anything in time, they could have timed detonations, etc, etc, etc.
And, could someone remind me of the type of bomb that doesn't seem to have an explosive blast, it just spreads a particular type of radiation that kills every living thing (excepting maybe rats and cockroaches), and leaves the buildings standing for possession by the invaders? It is a neutron bomb? Of course, true terrorists probably wouldn't use one of these, as the destruction is as important to them as the killing.
Sue