A land-carried nuclear weapon, to be effective, would have to be smuggled into a central location near the target. Detonating it in the middle of a river would be horrendous, but would result in considerably less damage.

Bad news: unlike the air bursts of Hiroshima and Nagasaki, a ground burst in a city or elsewhere would create much more fallout, and our country has forgotten everything it ever knew about fallout, which is actually relatively easy to defend yourself against--if you know what you're doing. I've got a nice set of radiation detectors and dosage meters, bought surplus. How about you?

Good news: we are apparently ramping up our detection capability far beyond what we had before. Maybe we can stay ahead of the game. I don't know how well the detection technology works, and the government sure as hell isn't going to tell anybody.

Up until now, the "Bomb Cops" operated under the scenario that a terrorist would make a threat, and the forces and detection equipment would be deployed to the threatened zone. Now, with the liklihood that an attack would come without warning, a much more diffuse detection protocol has to be developed.

More good news: any nation on Earth--ANY nation--which suspected that there were terrorists inside its borders planning a nuclear attack on the U.S. would kill said terrorists in an eyeblink. Given the fate of Afghanistan after the loss of two skyscrapers, one can only imagine the fate of a nation held responsible for harboring people who busted an A-Bomb loose in some American city. Therefore, any terrorists doing something like this would have to be trying to keep it secret from not only U.S. forces, but from pretty much any government that could even remotely be blamed, which hopefully limits their resources.