BEHIND the German empire that emerged in 1870 as one of the most powerful states in the world, stretched a centuries-long, haphazard process of small political entities coalescing into larger ones. Until its ultimate stage the process lacked the goal of a single, all- embracing nation, but over generations centers of political power formed and institutions and attitudes developed that eventually made unification possible. An important advance on the road to nationhood occurred when the ruler of one of these power centers -- Frederick II of Prussia -- more than doubled the size of his state and began to challenge Austrian supremacy in Central Europe.

Like Bismarck a hundred years later, Frederick succeeded through intelligent, flexible diplomacy, coupled with the willingness to use force, in the service of policies that were ambitious but never unrealistic. Frederick was not unusual in regarding war as an appropriate and necessary instrument of international relations, but he differed from other rulers of his day by repeatedly going to war against opponents far stronger than he. He trusted in his army, his understanding of Prussia's strategic possibilities, and in his possession of ultimately military as well as political authority. That he controlled the administration and policies of his state and also led his armies in the field enabled him to make decisions more rapidly and surely than could generals who were responsible to distant monarchs and councils of state. Even under these conditions war remained a gamble, but one that seemed worth taking. War was not the only reason for the rise of Prussia, but the state could hardly have become a significant political force without it.

Christopher Duffy has written an interesting, informative book on Frederick's army and his manner of using it. A lecturer at the Royal Military Academy, Sandhurst, and a well-known writer on the history of war before the industrial revolution, Duffy has a good sense of the period and a thorough command of the literature. He treats issues and personalities, including Frederick's unusual combination of strengths and weaknesses, with understanding and a refreshing matter-of-fact pragmatism. Frederick's military biography is preceded by an outline of the early history of Prussia; the biography itself is strong on battles, less detailed on the king's strategic concepts, and very slight on Prussian military institutions, organization, and finance. The best parts of the book are the careful reconstructions of the many battles Frederick fought in the 46 years of his reign. Well-drawn if rather small maps further help the reader unravel the complexities of 18th-century fighting.

BUT WAR is more than tactics. The strategic characteristics of the campaigns, the political and economic considerations on which they rested, even the place of a particular battle in the overall strategy of a campaign, do not emerge with the same clarity from Duffy's treatment. Much more might be said, for instance, about the problems of coalition warfare on both sides during the Seven Years War or about the tensions in the same conflict between Frederick's operations, which were marked by the spirit of the offensive, and his necessarily defensive strategy. The author's undoubted expertise, which shines in his treatment of tactical details, does not result in an interpretation that encompasses the whole of Frederick's military life from small-unit administraion to grand strategy.

This raises a general question: since military institutions and policy are so closely embedded into their non-military context, can even historians who are primarily interested in war analyze their subject without looking beyond it? A reference to the history of this country may clarify the point. A "military life" of George Washington is certainly conceivable, and many books have, of course, been written on that subject. But an interpretation that limits itself to purely "military" factors can never do justice to Washington's generalship. Washington not only had to fight the British, his operations had to respond to and overcome such varied pressures as the power of the Continental Congress, the economic and political conditions of the 13 colonies, short-term enlistments and limitations on the use of the milita.

In the same way, Frederick's wars can be understood only in the light of his political ideas and his diplomatic maneuvers, and his operations and tactics only if the social, economic, and technological conditions of 18th- century Europe are fully taken into aount and integrated into the analysis. Duffy's book is good of its kind; but for readers who are interested not only in the tactics and drama of battles but also in their social and political characteristics and their strategic function the book leaves out too much. A more accurate if less attractive title for it would have been: Scenes From the Military Life of Frederick the Great.