Trita Parsi explains why sanctions are not responsible for the current negotiations with Iran:

In reality, it was neither the sanctions nor Iran’s centrifuges that produced the current breakthrough. The diplomatic opening came about for the same reason it did during the Cuban Missile crises: Both sides compromised. Tehran stopped advancing sensitive parts of its program and agreed to greater transparency. And Washington finally accepted enrichment on Iranian soil in the November 2013 interim agreement.

That is correct, but it should make us wonder why so many Westerners feel compelled to credit sanctions for the diplomatic progress that they have almost certainly blocked and hindered for most of the last decade. Here are a few possible answers. Many pro-sanctions hawks are convinced that authoritarian regimes will compromise only under duress, and they take for granted that Iran had to be coerced into making concessions on the nuclear issue. To admit that coercive policies are ineffective and even counterproductive in these situations would be to concede that their views on Iran policy were misguided from the start. There is also a widely-shared aversion to compromise with such regimes for fear of being labeled as an “appeaser.” I suspect this is why administration officials make a point of emphasizing that sanctions made diplomatic progress possible. This way they can better deflect the inevitable charges of “surrender” and “weakness” that hawks have thrown at them anyway.

There is also a tendency for supporters of bad policies to try to justify having done the wrong thing by pointing to something that has happened later on as “proof” that the bad policy was, in fact, a successful and wise one. If sanctions were imposed before Iran reached a deal with the P5+1, then sanctions will be given credit for causing this result despite the fact that they have been and remain an impediment to resolving the nuclear issue rather than an aid. This is the same sort of thinking that credits the Iraq war with Libya’s agreement to dismantle its weapons programs because the one occurred after the other. The claim isn’t true, but it is a very useful thing for advocates of a bad policy to say after the fact.

Parsi is right to note that “sanctions were needed to pacify domestic political forces in the United States and to give Obama the space he needed to pursue diplomacy down the road,” but as have seen over the last few months domestic political opponents have not been pacified in the least. On the contrary, Iran hawks have used the official “sanctions worked” line to demand additional sanctions, which everyone involved understands would sabotage negotiations. Buying in to the false idea that the sanctions have been essential to diplomacy with Iran has left the administration with no more space to pursue a comprehensive deal than it would have had otherwise, and it potentially makes it that much harder to provide sanctions relief as part of such a deal.

One danger of perpetuating the myth that sanctions were essential to Iran diplomacy is that it will wreck diplomacy with Iran. Another danger is that future policymakers will cite this episode in future sanctions debates as “proof” that sanctions can lead to desirable outcomes. This will reinforce the bad habit of believing that other states can be forced to give up non-negotiable things as long as enough pressure is applied, and as Parsi observes that will produce standoffs and possibly even armed conflicts over issues that could have been resolved much earlier on.