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.