Iran has a network of smugglers throughout Europe and North America. Most of these people are freelancers, but the Iranians pay well, and use their network to quickly and quietly move stuff, and people, around. In this way, the Iranians have managed to keep thirty year old, American made warplanes and missiles, in working order. A network that can move aircraft parts, can also move bits of nuclear weapons technology.
Sanctions that merely made life difficult for the mullahs led them to clandestine methods of overcoming those sanctions as much as possible. The networks established to overcome sanctions are now used to gather nuclear technology.
I mentioned that Iraq did much the same to help support the current insurgency. Underground sanctions-breaking networks built in the 1990s gave the Iraqi Baathists the experience they needed to finance an insurgency and terrorism campaign after March 2003.
And today, when we consider how we might stop Iran from going nuclear, consider that Iran has operated under santions of one form or another for more than a quarter century and they still have missiles and nuclear programs and lots of other deadly goodies. So anybody thinking that mere sanctions will drive Iran to their knees are fooling themselves.
Blowback is a bitch--even for soft options, including sanctions. Although as I've written this before, I think that the fixation on "blowback" is an overblown worryfest designed merely to forestall any action at all. Blowback is simply a name given to the fact that we can't find silver bullet solutions to our problems. Just solving them one at a time as they arise is tough enough. Don't expect the live happily ever after. Just expect to have made the problems we face a little smaller at best--or at worst just a little different and a little later.
Still, it is useful to remember that blowback can come from hard actions or soft actions--or even inaction, of course.