Defense planning unavoidably must be in the nature of a mystery tour. The inability to know the future is a permanent condition for defense planning, but it is one with which we must cope. This monograph by Dr. Colin S. Gray explores and examines the implications of our irreducible ignorance about the future. His purpose is to identify an approach to this critically important subject of security that leans heavily upon what we can and should know about the past and present, in order to anticipate future conditions. The monograph finds that, although the past does not repeat itself in detail, there are profound persisting reasons why it is repeated approximately in the challenges and dangers that security communities must face. Dr. Gray concludes that notwithstanding the facts of contextual change in strategic history, the "great stream of time" from the past, through the present, into the future commands critically significant continuities in history that yield an approach to the future...