Preemptive war is justified by an imminent threat of attack, a clear and present danger that the country in question is about to attack you. In such a case a preemptive attack is recognized as justifiable.The article offers the example of Israel's first attack in the face of massing Egyptian and Syrian troops. Such an imminent threat is the absolute minimum moral justification for waging war.
Regardless of any other evaluation, it is obvious that any nuclear threat of Iran's towards Israel is neither imminent nor present, existing as it does years in the future.
(h/t to Lawyers, Guns and Money)