I should not post this but:

A small nuke on a small plane, detonated over a large city, would have the greatest effect. Air bursts are more effective than a ground burst as any web site about nuclear war will reveal.

There may or may not be soviet briefcase nukes in the US. They may or may not be in the hands of radicals. They may or may not have access to small airplanes.

They do have access to a lot of money that can get them things we do not want them to have.