What are you going to do then?
The Israelis don't let just anybody drive big heavily-laden trucks into their country, unlike some countries I know. :)
"A nuclear launch is an easy scenario. What happens if a weapon get carried in by truck with no easily descernable point of origin?
What are you going to do then?"
The origin of source for the nuclear material can be traced and do have spies in almost every nation on the globe. However, the difficult part of who to punish in the event of a terrorist nuclear attack is that this could come from Ukraine or other former Soviet states that sold nuclear material on the black market.
Ukraine itself made an unprecedented statement that 12 of it's ICBM's were 'unnacounted for'which wound up in Iran. I would assume nuclear material from Ukraine and other Soviet states (raw uranium & plutonium) have also got into the wrong hands.