I've remarked before on a number of occasions that when evaluating the criteria for a just war, a ton of gravity doesn't equal an ounce of certainty.
The "one percent doctrine" is when a one percent chance of a grave event like a WMD attack is responded to as though it were a certainty. Basically it represents an attempt to make gravity and certainty mutually fungible as a basis for action: to make significant enough gravity act as a substitute for certainty.
Beware lasting, grave, and certain hypothetical scenarios. Beware arguments that are chains of a thousand subjunctives, but seem in summary to be logical works of the utmost realism and clarity. Be aware that soldiers are taught just war doctrine, while their civilian commanders are taught realpolitik.
Zippy's choice phrase, the certainty-substitute, needed currency back when Condoleeza Rice was, with utmost gravity, declaring that we couldn't wait for a mushroom cloud over DC to act.
The One Percent Doctrine Zippy summarizes could have as its motto "I can't believe it's not certainty!" It is double-edged, though only its hawkish edge has been doing any cutting. While there might be a one percent possibility Tinpot Dictator A will give WMDs to Terrorist Group B, there is an equally hypothetical possibility that post-invasion chaos will land said devices in Group B's hands anyway.
That one percent is a fig-leaf of fear used to clothe otherwise unacceptable arguments. That that leaf wasn't lifted earlier is a misfortune of immense proportions.