Why from within, why not from a couple of miles off shore?
My opinion is no one will bother to cross a border, just get a boat or ship, get close and either die with the bomb or use cruise control to send it in as someone takes another boat out.
Money is a big factor. Why develop a nuke when you can afford to buy it?
It is much easier to come across Mexico — boats hanging around NY might draw attention.
Texas Gov. Rick Perry Claims ISIS Terrorists May Have Already Entered U.S. From Mexico
countries with bomb grade fissile material know/should be smart enough to know, it's like a fingerprint and can be traced right straight back to them if it's used after they sell it whether or not we actually do anything about it after the fact is another story, but we will know
Why waste time and money on a delivery system that might be thwarted by air and missile defenses? They could quite confidently walk the critical components across the open border, buy or make he rest here. Our laughable border security removes virtually all of the operational risk associated with that approach. Probably most or all of the components are already here.
“Why develop a nuke when you can afford to buy it?”
You don’t need a nuke. If Little Boy and Fat Man had failed the idea was surfaced to fly over the Ruhr Valley and spray the radioactive material on the land. It would have made the Ruhr uninhabitable. A conventional bomb salted with radioactive material could spread that material over a mile or so of a city. Depending on where, the effect would be the same as a bomb. The entire place would be abandoned like Chernobyl today. (If it happened anywhere in California, the libs would flee the state in terror.)