The current president of Iran has announced that the destruction of Israel is an important part of their agenda, and that's unacceptable, and the development of a nuclear weapon, it seems to me, would make him a step closer to achieving that objective.
As I reported in my post on January 1 Israeli intelligence believes it will be impossible to stop Iran's nuclear program if concrete action is not taken by March. I think it is unlikely that the Security Council will agree on any sort of military action if Iran ignores a resolution, something Iranian President Ahmadinejad has made clear his country would do:
The main question is whether or not the Europeans will go along with an American-led action this time despite what has happened on Iraq. I can only hope that Prime Minister Blair, President Chirac, and Chancellor Merkel all understand that Iran is not Iraq and there is no doubt about the threat posed in this case. The more unified the action, the broader the coalition, the less the likelihood of serious retaliation or an escalation into a larger regional war.