If RMA (revolution in military affairs) was the acronym and concept of choice in the U.S. defense community in the 1990s, so preemption has threatened to supercede it in the 2000s. The trouble is that officials and many analysts have confused preemption, which is not controversial, with prevention, which is. In this monograph, Dr. Colin S. Gray draws a sharp distinction between preemption and prevention, and explains that the political, military, moral, and strategic arguments have really all been about the latter, not the former. Dr. Gray provides definitions, reviews the history of the preventive war option, and considers the merit, or lack thereof, in the principal charges laid against the concept when it is proclaimed to be policy. Dr. Gray concludes that there is a place for preventive war in U.S. strategy, but that it is an option that should be exercised only very occasionally. However, there are times when only force seems likely to resolve a maturing danger.