George F. Kennan has had two careers. The first was as a diplomat. A conservative realist who was the first head of the State Department's policy planning unit in 1947, Kennan coined the word "containment" for American strategy toward the Soviet Union. But his principled criticism of what he considered an overambitious foreign policy reduced his influence in government from the mid-1950's onward, while giving him a new audience among the intellectual left. At the Institute for Advanced Study in Princeton, N. J., where he has been a scholar since 1953, he has spent the last half century as a historian, best known for "American Diplomacy: 1900-1950" (1951) and several volumes of memoirs. In "An American Family," Kennan, now in his 90's, turns to the link between national history and memoir-genealogy.

Kennan limits his scope to the history of the first three generations of his family in North America -- and limits it further by focusing on the generations bearing the paternal surname ("only some 3 percent of my own genetic inheritance would seem to have been derived," he acknowledges, "from the first Kennan to have arrived on North American soil"). The choice is influenced, one suspects, by his interest, manifest throughout his work, in the rural America of the colonial period and the early republic, which has provided him with a standard against which to judge modern American society. "From his Puritan vantage point," Eugene Rostow once wrote, "Kennan has excoriated what he regards as the vulgarity, materialism and bad taste of the American culture; its deplorably simplistic and irrational politics . . . and its increasing alienations from the true sources of moral purity -- the life of small agricultural communities." Kennan's epilogue begins: "This concludes what I picture to myself as the innocent stage in the Kennan family's history. It was innocent mainly in the sense that it involved no residence in any large urban community." He adds: "Reviewing what I know of their lives, I cannot find in them any instances of deviousness, sordidness, or cynicism. . . . And trying to view them as individuals, I am unable to find among them a single weakling, a single physical invalid."

If these passages appeared at the beginning rather than the end of the book, few readers might be tempted to continue. But Kennan's virtues as a historian rescue his narrative from the virtues of his ancestors. Because the documentary evidence for this as for most families is limited, Kennan is forced to construct scenarios that plausibly explain minor historical mysteries: why and how James Kennan of Dumfries, Scotland, born between 1715 and 1720, arrived in Massachusetts before 1744, or why in the 1790's his son George moved his family to Vermont.

That these mysteries are minor is what makes them familiar and interesting; for most people, after all, the highlights of life, from births to marriages to deaths, are precisely those preserved in family records. In his conjectures, Kennan is guided both by a shrewd sense of character and by an impressive knowledge of the folkways of his New England ancestors. The former is evident in his inferences, from scant data, about the state of the "rather sudden" marriage of George Kennan and his slightly older wife Abigail in the 1790's. Was it really "young love," he asks: "For a girl of a simple country family of that time to reach the age of 24 and be still unmarried was to find herself in a precarious situation." His grasp of New England traditions enables Kennan to solve other mysteries, at least to his satisfaction: "The departure and disappearance of Aaron Booth must have left the Kennans with two houses on new property they were taking over. And this served more than one purpose. For it was a firm and longstanding rule of New England society that when a son married, he must never try to take his bride and progeny back into his parents' home for regular residence."

Kennan's conjectural history of his early ancestors (and it is conjectural, despite the use of "plainly" and "it is clear") is enriched and varied by vignettes about striking or influential neighbors and by discursions into regional history and communal politics. What might have been a dry discussion of the debate between liberal and conservative Calvinists in New England has practical consequences for Thomas Kennan, an ordained Congregational minister: "And one of the positions of the strictly Calvinistic conservative faction was that no one who had not had some form of formal academic training in the field of theology ought to considered eligible for a pastoral position. Thomas Kennan, of course, lacked any such training. So it would not have been in any way surprising had the influence of this powerful faction been brought to bear against his being given any pulpit within the state of Vermont."

A modest book that succeeds on its own terms, "An American Family" might be of interest to readers curious about their own history, as well as to students of New England tradition and one of that tradition's major contemporary representatives, George F. Kennan.