(NSA) The US govt. does have a cyber intelligence agency which is capable of infiltrating hostile nations computer systems and figuring out where exactly missiles are being kept.
(NRO) Also the ability to spy from some very advanced and very expensive satellites.
(CIA) Ground assets to acquire more intelligence and eliminate targets if necessary.
(US Air Force) Capable of infiltrating enemy skies and delivering bunker busting bombs such as the Massive Ordnance Penetrator or nuclear bombs / missiles much larger than any other country. Our nukes have actually been tested for 50+ years and we know they work.
http://en.wikipedia.org/wiki/Massive_Ordnance_Penetrator - Mainly developed for destroying hardened bunkers housing nuclear weapons.
(US Navy) Submarines with Trident missiles which carry between 1 and 8 warheads apiece and their locations are unknown but you can bet that they are strategically placed outside of potentially hostile nations, meaning we can nuke anyone faster than they can nuke us. Also our radar, missile detection, and anti-missile systems are much more advanced than any other nation.
Yes, Iran might nuke Israel, but they know for a fact that the United States will spare not expense invading them and kill everyone responsible for the attack and many more.
North Korea main reason for acquiring nuclear weapons seems to be as a deterrent against invasion. You don't invade a country with nuclear capabilities unless you take out the nukes first, it's just too risky for your ground troops.
This is an image of an old Peacekeeper missile with multiple warheads. This missile has since been retired but still leaves me in awe. http://upload.wikimedia.org/wikipedia/commons/5/5f/Peacekeep...
It would be a huge mistake for any country to launch a nuclear missile towards a foreign city. The world (mainly the US) would jump on them and whoever ordered that nuke would be dead very quickly.
The big problem with nuclear bombs is ensuring our borders and ensuring customs does a good job. A nuclear bomb has the problem of not leaving any trace or signature. How would you investigate it? There wouldn't be anything to investigate except where that plane or cargo ship came from.
If a plane from North Korea explodes with a nuclear bomb on it, then would we even be able to tell which plane exploded? The entire airport would be vaporized.
I honestly don't know how it hasn't happened already. People want to get rid of the NSA but maybe they're the only agency actually preventing this scenario by tracking most nuclear bombs. I can't imagine that all incoming cargo is screened sufficiently before it arrives in the US, it would truly be devastating for this to happen, all imports would be immediately halted. Maybe cargo ships or ports that export have some sort of device to detect nuclear weapons (I'm not sure how easy the are to detect).