It seems like I've written about this same issue many times before in the contexts of both Iran and Israel-Palestine. It is also largely applicable to Iraq. People in a country (or in any other context, for example a family) may have their differences. They may not like each other. They may want to see change. But you do not capitalize on those differences by threatening them. The only thing that will do is bring them together in unity against you. Of all the stupid things I've heard and read about the possibility of military action against Iran, the most irritatingly stupid has been the oft-repeated idea that the young people in Iran don't like the conservative clergy who run the country and will therefore welcome an invasion or, in the face of Western pressure, will overthrow the government. My god, have we not heard this before?
The Iranian clergy had real reason to worry about their future position after the 1998 election, and consequently they allowed some moderation in policies that they otherwise would not have. Our constant belligerence towards Iran since 2001 has put an end to that and has seen a moderate president Khatami replaced by an extremist Ahmadinejad, and talks of reform replaced by talks of nuclear weapons.
When Khatami won the 1998 election, the Clinton administration, after much deliberation, decided that the proper response was to do nothing. Any action taken in support of the reformists would only have served to taint them with the appearance of Western influence. It was a fight that belonged to the Iranian people, and one where the side we favored had a good chance of success. Patience was the proper course.
That opportunity for internal reform in Iran has now been squandered, and reformists will need years to recover. I can't say whether we can afford to wait years for them. But I can say with some confidence that we can choose to either maintain an aggressive posture (as is the present course) or hope for internal reform in Iran. There is no option that will allow both. We are rallying reformists and conservatives alike to the flag. If we back off and hope for reform, there is no guarantee that Iran won't get the bomb. In fact, Iran probably will. But if they really liberalize, and I think they might, even with the bomb, that is a far better outcome and a more permanent solution than will ever be achieved through an aggressive approach.