This really isn't that difficult. A "preemptive" war is when you preempt an imminent attack by attacking first. Israel preempted a first attack by the massed Arab armies in the Six Day War. As a matter of international law of war, there is absolutely no question about one nation's right to attack first to preempt an imminent attack. If an American submarine had come across the Japanese fleet launching planes on December 7, it would not have to have loitered around until bombs were actually dropped on Pearl Harbor before it sank the carriers.
What the United States asserted in Iraq -- what Charlie Gibson would have meant if he understood the Bush Doctrine -- was a controversial right to launch a preventative war. A preventative attack is meant to defend the attacking nation by preventing a probably enemy from getting any where near to attacking. As Dick Cheney said, we didn't want to wait until an Iraqi attack was imminent. We took that position because 9/11 had taught us that we weren't willing to wait to be attacked, even if the attack did little strategic damage.
One of the lessons of the internet is that the left is every bit as ignorant as the right, but that ignorance is more annoying when it accompanies an assumption of superior knowledge.