Mutual Assured Destruction kept the US and the USSR from pushing the button. If Iran's mullahs were to be insane enough to attack Israel, the Israeli nuclear response would be crushing. No one knows exactly how many nukes Israel has, but rest assured it's a bunch. Probably plenty to vaporize most large Iranian cities. The big question: does MAD work when yer dealing with religious fanatics?
MAD requires rationality.
Does MAD work for religious fanatics who are downtrodden, destitute and very angry? Probably not.
Does MAD work for religious fanatics who are rich and powerful and who control an entire nation? Most likely, Yes.
"The Mullahs of Iran, even amongst the hardliners, are by no means suicidal, and place great value on their unyielding grip on power. They know very well any aggression towards Israel will have serious consequences for longevity of the Islamic Republic and hence their monopolistic rule."
This article is a great analysis on the Iranian nuclear position.
"Many important details of Iran’s nuclear program simply do not form part of the mainstream debate. Contrary to the myth surrounding Iran’s nuclear program, an informed and reasonable observer will quickly realize there are indeed two sides to every coin, and that claims of the Iranian nuclear ‘threat’ are arguably more political than factual; that the United States’s unbending stance in rejecting Iran’s offer of negotiations without preconditions – United States insists uranium enrichment must be totally suspended – has served to exacerbate a dire situation. The fact is, Iran is in compliance with international law in practicing its legitimate right to develop nuclear energy technology in accordance with the NPT. Let us explore some additional facts."