I gotta throw some cold water on this. Everyone seems to think of nuclear weapons as requiring intercontinental or hyperson delivery.

Remember, close only counts in horseshoes and atom bombs.

Let's just hypothesize a bad actor with medium supply of megaton warheads.

You put a couple on a private Learjet (cheaper than a cruise missile with delivery system) and trigger it on final approach to your target airport. Airburst.

You put a several on fishing trawlers that slide into harbor towns.

Maybe you put one in a pig (a cleaning device for international running natural gas pipelines.)

You bury a couple in garbage barges full of metal for recycling or some such.

I don't mean to be giving bad guys a lesson here, but it's ridiculously easy to do to any country that does not have iron curtain type borders (USA anyone?)