[Spktyr]

Missile not required. Just mount the warhead on a boat and sail into the harbor. Our harbor security is crap.

[Malcolm Reynolds]

Maybe getting to the port itself, but not beyond. I'm guessing we also have ways to monitor nuclear material at sea.

https://www.dhs.gov/blog/2017/04/06/new-radiation-detection-system-operational-major-us-port

[jonrick46]

We have satellites with radiation detection equipment. There is no way a nuclear device can be concealed with any amount of shielding on a boat. A submarine is a different matter. I think we can track the subs from space.