The problem with deterrence - apparently sometimes forgotten by our former presidents - is that it is not static, but a creature of the moment, captive to impression, and nursed on action, not talk. It must be maintained hourly and can erode or be lost with a single act of failed nerve, despite all the braggadocio of threatened measures. And, once gone, the remedies needed for its restoration are always more expensive, deadly - and controversial - than would have been its simple maintenance.