"One cannot fashion a credible deterrent out of an incredible action"

This quote by Robert McNamara is referring to the concept of prevention, which is using dangers to prevent a foe from taking an activity. McNamara is stating that it is not possible to create a legitimate deterrent if the activity being threatened is not reputable. In other words, if the action being intimidated is not viewed as reasonable or feasible, then it can not be utilized as a deterrent. This is due to the fact that the adversary will certainly not take the risk seriously as well as will not be deterred from taking the action. Therefore, McNamara is highlighting the significance of making certain that the action being intimidated is reliable in order to create an effective deterrent.