The tons of drugs that are smuggled into this country every year somewhat undercuts the argument that we can stop it at the border. Besides if you are setting it off in the San Francisco bay or the Hudson River you don't really need to go through customs. Primitive bombs are big but they are not so big that they couldn't fit in moderate sized pleasure boat and shielding isn’t really that complicated if you have room and can handle the added weight.

We stop only a fraction of the drugs that come into this country, that wouldn’t really be good enough with nukes.

Hell you could simply put it in a cargo container and have it set to go off if it is opened. Smuggling a nuke into the country would be risky but it wouldn't all that hard and 911 taught us they can find people who are willing and able to pull the trigger even if they are sitting on top of it.

As for the Cold War I think you are the one misunderstanding. The Cold War mindset is not "The fear they can hurt us" I am not even sure where you got that idea from but you can't simply just make such things up and assume the rest of us will accept them as fact. The Cold War mindset more then anything else is the mindset that having more and better weapon systems can protect us and that MAD will stop anyone from pulling the trigger. The Cold War mindset was that you didn't have to actually use these more and better weapon systems because simply possessing them would inspire such fear in your enemy that he would never attack you. Not sure that works with international terrorist organizations. If we find out weeks later that a nuke set off in Atlanta was made with uranium from Iran are we going to turn around and nuke Iran? If the answer is not 100% yes then MAD doesn’t work.