Jeffrey Goldberg passes along this quote from Yossi Klein Halevi:
In the last few years, Israelis have been asking themselves two questions with increasing urgency: Should we attack Iran if all other options fail? And can we inflict sufficient damage to justify the consequences?
As sanctions efforts faltered, most Israelis came to answer the first question affirmatively. A key moment in coalescing that resolve occurred in December 2006, when the Iranian regime sponsored an "International Conference to Review the Global Vision of the Holocaust," a two day meeting of Holocaust deniers. For Israelis, that event ended the debate over whether a nuclear Iran could be deterred by the threat of counter-force. A regime that assembles the world's crackpots to deny the most documented atrocity in history--at the very moment it is trying to fend off sanctions and convince the international community of its sanity--may well be immune to rational self-interest.
Bold added. Let's think about this. Why rational reasons might Iran have for building nukes?
1. Israel has a bunch of them, and Iran is scared of Israel for reasons described in Halevi's article: they quite often act rashly, and Iran wants to deter that.
2. Iran is a Persian Shia state in a largely Arab Sunni region. Their largest regional competitor (other than Israel) is Saudi Arabia, and they are nervous that Iraq will once again become a major regional power. Iran wants to deter other regional states, and preserve its status as a big player in the Middle East.
3. The U.S. military is on either side of them in Iraq and Afghanistan, and has been rattling its saber in Iran's direction since 1979. Iran wants to deter a U.S. invasion.
4. The pattern of recent history is that if you do not have nuclear weapons and challenge the U.S., you run a high risk of being invaded or attacked by the U.S. or Israel (Iraq, Afghanistan, Syria, Balkans, etc.). If you do have nukes and challenge the U.S., you run a zero risk of being invaded or attacked by the U.S. or Israel (Pakistan, N. Korea). The lesson is obvious and clear: if you want to deter international interference, get nukes.
As comforting as it might be to think of Iran as irrational and irresponsible (since that justifies aggressive action of the sort discussed in Halevi's piece), the truth is likely the opposite: if Iran is pursuing nukes, it is for very rational reasons.