The concept of "preventive" war is nonsense. All wars are preventive. Some wars are designed to prevent an enemy from taking control. Others are designed to prevent an enemy from remaining dominant. More than a few are designed distract and prevent the population from thinking…and then removing from power an incompetent leader. Some wars are designed to prevent the population from realizing that the leader has been carrying out a fraudulent and failing foreign policy. An attack on Iran at this time would certainly come under that heading. Some wars are designed to prevent a subject population from achieving independence. Others are designed to prevent an emerging power from achieving its "place in the sun." An attack on Iran would come under that heading as well. Some wars are designed to prevent things that no one was trying to achieve in the first place. Many are designed in part to prevent loss of face. Some are designed to prevent the loss of profit.
War is war, and those who start wars are criminals who should be held responsible. Those who make war against civilians in a policy of collective punishment are also criminals. That is why Americans reacted with such anger at 9/11: 3,000 cases of immoral "collateral damage" blocked any rational consideration of al Qua’ida’s goals (e.g., removal of U.S. troops from Saudi Arabia, which soon thereafter occurred, and just treatment of Palestinians). Those who incite war or arm both sides to prolong war or provide weapons specifically designed for slaughtering civilians are also criminals. It really does not matter how many cute little adjectives you put in front of war: starting a war is immoral, criminal behavior. It is simply wrong.