If Iran decides to use nuclear weapons, the odds are that the Supreme Leader and company would start with Israel. A "respected Washington think tank," the Center for Strategic and International Studies (CSIS), made an educated guess about what would happen. It's an "estimate," since Israel isn't saying anything about the megaton nuclear bombs they're rumored to have, and Iran's policy, when it's not organizing "Death to Israel!" rallies, is that nuclear program is peaceful. (There's a fairly detailed, if somewhat dated, discussion of Iran's program at Global Security's website.
Lots of people would die.
However, about three weeks after that particular jihad started, Israel would have serious rebuilding to do in Tel Aviv and Haifa, among other places, and over 80% of the people it had before the destruction started.
Iran would have to stage its "Death to the Great Satan America!" rallies somewhere other than Tehran and Tabriz, and I suspect that Natanz and other parts of what used to be Persia would require extensive international aid before survivors could move back. Iran might have 75% of the people it started out with. The CSIS thinks that, for practical purposes, there wouldn't be an Iran any more.
I'm not so sure. People as a whole are resilient. I think the deciding factor is whether surviving Iranians would want to get back to the old hate-the-Jews-and-Yankees Islam, or try something else.
Even best-case estimates don't look good in the CSIS study, as reported:
|Low Estimate||High Estimate|
Prospects don't look too good for Syria or Egypt, either, if the leaders of those countries decide to try a do-over of the Arab world's 1967 effort to push the Jews into the sea.
I sincerely hope that a war like this never happens.
The appalling death toll would suggest that Iran's leadership wouldn't start a war that would kill so many of their subjects. On the other hand, the ayatollahs might think that they're doing Iranian Muslims a favor: from what I understand of Islam's lunatic fringe, dying in a Jewicidal jihad would earn them a place in the Las-Vegas-style afterlife we've heard about - the 72 virgins and all that.
All things considered, I'd say it would be a good idea to reduce the odds that Iran gets nuclear weapons.
There's more at " 'Israel could survive nuclear war' " (Jerusalem Post), and "Report: Iran Would Suffer Up to 20 Million Casualties in Nuclear War With Israel" (FOXNews.com).