I would say that sanctions played an important role in the end of the apartheid regime in South Africa.  And it wasn't that the sanctions trashed the South African economy (though they did have strong effects), but the near universal international disapproval that came with the sanctions was a big factor, I think.

You could try and cite South Africa as a successful case, but that was really a very different situation; sanctions didn't induce bottom-up opposition to the regime (that already existed), they helped break South African elites' willingness to clamp down on the opposition.

Didn't read carefully enough the first time -- but I think you're wrong.  Bottom-up opposition already exists in Iran as well.  You don't have to look far to find accounts of violently oppressed protests.  And breaking the Mullahs willingness to clamp down on the opposition is precisely what is needed. 

Will sanctions have that effect?  I think that if such sanctions were as close to universal as the South African ones, they very well might.  But I also think that the chances of that kind of consensus, and those kinds of broad sanctions are very low. 

Matt clearly says externally-induced economic collapse.  Thus the French Revolution and the collapse of Soviet Russia are irrelevant.  South Africa was different in many ways, including that it was a (whites-only) democracy, so the ruling party couldn't ignore the white-owned companies which suffered. 

Sanctions have a terrible track record against dictatorships.  Mostly because they hurt ordinary people and businessmen, neither of whose support the dictatorship counts on to stay in power.  All a dictator needs are their army generals and a small elite cadre of supporters.

Another reason for the usual failure of sanctions to effect political change is that rulers threatened by sanctions generally take action to avoid having them imposed.  If sanctions are enacted, it means that the ruler believes he can handle them.

 http://www.atimes.com/c-asia/AA08Ag02.html 

"Revolutions of rising expectations occur when people begin to expect more owing to improvements in their lives. And such optimistic attitudes sometimes lead them to make demands that neither the economic nor the political system is able to meet. That frequently results in a crisis that can lead either to the transformation of these systems or to the demobilization of the groups making such demands. But in either case, optimism that goes beyond the capacity of the country to cope can create instability." ...Paul Goble

This paragraph is a quck precis of the idea MY is referring to. The link is for credit, the article not really relevant.

Better:

http://en.wikipedia.org/wiki/Crane_Brinton

Crane Brinton apparently originated the idea;Wiki Entry

Googling the name brings up essays on specfic historical revolutions

I tend to agree with Richard about the emptiness of the sanctions talk. The days when the US could lead a sustained and disciplined Western sanctions regime against a large and important country like Iran have passed. The sanctions against Iraq unraveled in the late nineties, and US policies since then have only further undermined US credibility and moral authority.

And the post-Cold War policies of the past fifteen years, driven by triumphalist ideological enthusiasms, have weakened the US position in other ways, and helped limit its options in dealing with Iran. Unrelenting Western provocations, plunder and subversion inside the Russian sphere of influence, for example, and the US's seemingly uncurable, habitual hostility toward the Russian state, have Moscow worried about a new Cold War. This has helped drive a wedge between the two countries, and has probably wrecked any chance of a coordinated Russian and American approach on Iran.

However, I don't share Richard's fatalistic attitude about the eventual acquisition of nuclear weapons by Iran. That outcome can be avoided with a change in US Iran policy, and a willingness to deal directly with Iran on a broad range of regional issues. But this change in policy can only come from below, and will require a campaign to educate the American people, help them see where their true interests lie, and create political pressure on the extremist US administration and other sectors of the foreign policy establishment.

At the present time, however, the administration appears hell-bent on regime change in Iran, which leaves it with only two main types of options: the military options, and the other crackpot Hail Mary schemes for fomenting "revolution in the streets" of Iran. Both are absurd, for a variety of reasons. One of the most significant, however, is that Iran, unlike Iraq, is a true nation with an ancient history as a unified people. Iranians will likely respond as a nation to either of these two tactics, and will further weaken our position in the Middle east, endanger the lives of Americans, and isolate us globally.

There are disturbing indications that the key figures inside the administration, including the President, have adopted the lunatic fantasies of the Ledeenite wing, and now see Iran as a Hitlerian, expansionist empire, directed at its center by the evil "terror masters", who diabolically pull all the strings, and pay the bills, of global terror. The complextity of Iranian society and government are all buried under a caricature of the dictatorial Mad Mullahs brutally suppressing their people and scheming to conquer the world. And Ahmadinejad, a populist punk of limited powers, who is not even in control of the Iranian defense establishment, is elevated to the role of omnipotent dictator.

Recent Iranian overtures toward the US have been rebuffed, indicating that the rejectionists in Washington, and all who are threatened by any talk of a rapprochement between Washington and Tehran, are firmly in charge:

Iran has prepared a high-level delegation to hold wide-ranging talks with the US, but the Bush administration is resisting the agenda suggested by Tehran despite pressure from European allies to engage the Islamic republic, Iranian politicians have told the Financial Times.
The Bush administration is resisting pressure from its European allies to engage Iran directly over its alleged nuclear weapons programme rather than leave negotiations to the EU3 of France, Germany and the UK. Frank-Walter Steinmeier, the German foreign minister, raised this issue with Mr Hadley this week, and Angela Merkel, the German chancellor, is understood to have spoken about it with President George W. Bush.

....

Javad Zarif, the Iranian ambassador to the United Nations, stressed Iran’s willingness to talk in an opinion piece published by the New York Times on Thursday. He denied US claims that Iran had a covert nuclear weapons programme and said Iran was ready for intrusive international inspections.

Everybody should now be reading the new Sy Hersh piece in the New Yorker to get a sense of the administration thinking. It is depressing that the administration has changed so little, has learned so little from its errors, and been so successful in running the public through the very same rat's maze of disinformation and propaganda that worked so well in the run-up to Iraq. Surely once fooled is enough!

Now, given the gravity of the situation in the Middle East, and the potential for even worse, can't a case be made that Democrats should be talking about the Iran issue just a little bit more? But once again, the party seems to have drifted into an election-year obsession with domestic issues, and is positioning itself for another right-wing national security swindle and power-play. It's 2002 all over again.

I notice that while these events are going on, the regulars at America Abroad took another eight-day vacation from posting last week. The silence was only ended by a one-paragraph throwaway compliment to Max Boot of all people! There is a new article in the New York Review of Books describing the phenomenon of the the Democratic blogosphere - particularly the Daily Kos - and its role in promoting discussion and coordinated activism among Democrats. And TPMCafe is playing a very constructive role in this movement where such issues as immigration, health care and gay rights are concerned. But on the most pressing issues of national security and global affairs, where there is a vital need for massive injections of information and debate into the public sphere, it has again been missing in action. Discussion and debate of foreign affairs issues is sporadic, one-sided and unenlightening.