...not impossible.

A twin-engined aircraft, or even a Cessna [with a suitcase nuke onboard], flown from an obscure or unguarded airstrip in Haiti or Mexico
or [I'm horrified to say] Canada, towards a port city like Miami, Houston, San Diego, Cleveland, etc...
is [alarmly] feasible...
But going through US Customs with a nuke? Not likely now with geiger-counters
available to those men and women.

It's also [alarmingly] possible that
al-Qaeda terrorists [trained as frogmen] could take one of those suitcase nukes and swim out into the Suez Canal to intercept a US Aircraft Carrier navigating
that narrow body of water.
And [Saddam would also like this!]
another al-Qaeda team [trained as suicide commandos] could try and race their way into the USAF base in Turkey with a couple of trucks [one or two carrying such nukes]...I think Larry J's background would help to know if such an attack would succeed or not.
I understand that you were trained
in USAF armed-sentry skills, Larry...True?

Jacob