The purpose of a modern system of national defense is to make all-out war unlikely by making it instantly achievable. This system, like the theory supporting it, is known as ""deterrence,"" and it is the subject of Mr. Englehardt's fact-filled little book. Employing the extended image of a living creature, he discusses in turn every ""organ"" from the gigantic radar ""eyes"" in Greenland to the computerized ""brain"" at Cheyenne Mountain, Colorado, with chapters for each particular sort of ""limb"" -- missile hard sites, manned bombers of the Strategic Air Command, Polaris submarines. All this has been told about before, of course, quite exhaustively, and usually with much greater emphasis upon what it all might mean in terms of international power politics. Mr. Englebardt's chief contribution, therefore, lies in his careful attention to small details and the personalized approach towards the wary men and watchful machinery of the most intricate, deadly, expensive, and -- one hopes -- ultimately unnecessary military project ever devised.