To define future threat is, in a sense, an impossible task, yet it is one that must be done. In this monograph, Dr. Colin S. Gray explains that the only sources of empirical evidence accessible to us are the past and the present. We cannot obtain understanding about the future from the future. Dr. Gray draws noticeably upon the understanding of strategic history obtainable from Thucydides' great History of the Peloponnesian War. The monograph advises prudence as the operating light for American definition of future threat, and the author believes that there are historical parallels between the time of Thucydides and our own that can help us avoid much peril. The future must always be unpredictable to us in any detail, but the many and potent continuities in history's great stream of time can serve to alert us to what may well happen in kind.