The Blank Page, the Final Frontier : NONFICTION : UNDAUNTED COURAGE: Meriwether Lewis, Thomas Jefferson and the Opening of the American West,<i> By Stephen E. Ambrose (Simon & Schuster: $30; 496 pp.)</i>

Imagine that you are a student and that the registrar’s computer has been playing tricks with your course enrollment. You thought you were signed up to take the standard American history course, but the computer has placed you instead in a history class for ROTC cadets.

Things become puzzling when your instructor’s lively stories keep returning to the same theme: the proper behavior and philosophy of a good company commander. Although unexpected and often quite interesting, this preoccupation does not strike you as doing full justice to the rich meanings of American history.

In similar ways, Stephen E. Ambrose’s “Undaunted Courage” is a story on a big scale with meanings squeezed into a framework built on a considerably smaller scale.

There is no question that this is a good and readable story. Meriwether Lewis was an extremely interesting man and writer, and any book with the license to quote him at length carries a competitive advantage. Born on Aug. 18, 1774, on a Virginia plantation neighboring that of Thomas Jefferson, Lewis hunted, learned nature lore and served in the militia. Soon after Jefferson became president, Lewis moved into the White House as his private secretary. The two often discussed exploring a land route to the Pacific Ocean, and in 1803, Jefferson successfully persuaded Congress to fund the mission.

Lewis invited his friend, Lt. William Clark, to help him lead 26 men into a vast terrain, much of which had just become, with the Louisiana Purchase, America’s own.

After devoting five opening chapters to Lewis’ origins and relationship with Jefferson, Ambrose’s next 25 chapters take their shape from the story of the expedition: the trip up the Missouri River in a 55-foot covered keelboat and two small craft; the cold winter in what is now North Dakota, where they acquired the help of a captive Shoshone woman, Sacagawea; the move from keelboat to canoes before crossing the Great Falls of the Missouri; the strenuous crossing of the Rockies; the wet winter at Ft. Clatsop on the Pacific Coast; the rushed, anticlimactic return to St. Louis.

Repeatedly, Ambrose, a historian best known for his three-volume biography of Richard Nixon and a recent oral history of D-day, locates the meaning of these stories in lessons of universal military practice. At the Pacific camp, Lewis’ goals in maintaining order and discipline, Ambrose says, were “the goals of every company commander from the time of the Roman Legions to today.” Meanings reaching so far over time can stretch themselves pretty thin. “A good company commander looks after his men,” Ambrose writes, adding that Lewis was like “the head of a family,” evincing concern for his men matching “that of a father for his son.”

A family composed of men accompanied by one Indian woman, however, would seem to be a social unit calling for an awareness of the various meanings and workings of masculinity, especially when a Virginia gentleman undertook to lead a party composed largely of French Canadians. But the call for an analysis of culture and gender is one of several calls from the 1990s that Ambrose has decided not to answer.

In truth, if you concealed the title page and asked readers to guess the publication date of this book, estimates might vary considerably. The subtitle’s reference to the “opening” of a presumably locked-up West, the celebration of the “discovery” of lands long occupied by Native Americans and the use of the terms “braves,” “red men” and “squaws” suggest a publication date in the 1950s.

Other phrasings sound very much more like the ‘90s: rivers were “free of any pollutants”; Lewis was most likely “manic-depressive”; beyond the Platte River, the expedition entered “a new ecosystem”; Lewis was “sensitive and caring” toward his mother. Ambrose is, moreover, very much a celebrant of hindsight--in his phrasing, “after-action analysis”--and an enthusiast for the identification of “mistakes.”

And mistakes accumulated badly at the end. For those taken by human nature and its weird ways, the final seven chapters of the book, covering Lewis’ last three years of life, may well be the most intriguing.

When the explorers returned after a two-year journey of 6,000 miles, there was much celebration, for they had been presumed dead. But Lewis--the man who rose to every challenge in his crossing of a continent--was laid out flat by the conditions of his return. Preying on his apparent predisposition to mental illness, three prosaic elements of early 19th century American social and cultural life knocked the triumphal explorer off his feet: the difficulty of courtship, the irritations of administering a territorial government and, perhaps worst of all, the problems of preparing a manuscript for publication.

Upon their return, Lewis and Clark both went looking for wives and jobs. Clark got a wife; Lewis didn’t. Clark became the capable superintendent of Indian affairs for the Louisiana Territory; Lewis became governor of the same territory, trying to use his office to profit from the fur trade. An extraordinary degree of bureaucratic and factional cat-fighting circled around Lewis’ tour. As Ambrose puts it, “if he was a near-perfect Army officer, Lewis was a lousy politician.”

And then there was the problem of manuscript preparation. After signing up a publisher and illustrators and consulting experts in a burst of activity in Philadelphia, Lewis did absolutely nothing with the journals. Despite big expectations and big promises, Lewis-as-author went into major-league default and denial.

For any writer who has ever missed a deadline, this episode in non-publication shows Lewis at his most human and most in range of empathy. It cannot be a comfort, then, to know that Lewis, in mid-writer’s block, went mad. To rescue himself from financial ruin partly caused by the Madison administration’s refusal to pay some of his official expenditures, Lewis set off to Washington. He made it only to the hills of Tennessee, however, where he was found dead in an inn. While schoolbooks told us that he was murdered, Ambrose argues that he committed suicide.

By his failure to prepare the journals for publication, Ambrose asserts, “Lewis cheated himself out of a rank not far below Darwin as a naturalist.” In the observation of nature and the describing of specimens, Lewis was a remarkable fellow, alert and observant to beat the band. But if Lewis came up with an organizing idea, a concept anywhere near as powerful as evolution, the record of this breakthrough does not survive. The comparison of Lewis to Darwin tells us, instead, that we are reading a biographer who had become very fond of his subject.

Acknowledging that Lewis is one of his “heroes,” Ambrose argues that the explorer needs to be better appreciated. Lewis’ suicide “hurt his reputation” and deprived him, for instance, of the honor of having a river or another major geographical feature named after him. Those with more of an enthusiasm for word play and puns may notice what Ambrose did not: with a minor twist of misspelling, “Louisiana” Territory stands ready for transformation.

In the interior West today, we are doing considerably better than Lewis did in courtship and marriage and, as the current renaissance in western literature indicates, we are dealing very well with the challenges of preparing manuscripts for publication.

But in matters of governance, the tangled, factional, profiteering, rumor-filled and contentious politics of Lewis’ territory in 1809 seem entirely too familiar. Stymied to find a way to reconcile public interest with personal profit and locked in a struggle over the question of who will benefit from the region’s great resources and magnificent scenery, we are indeed residents of Lewisiana Territory.

Ambrose tells us that Lewis, in the spring of 1805, “turned his face west. He would not turn it around until he reached the Pacific Ocean.” The turning around, the looking back, the return to life in one’s own contentious home country, proved and proves to be the hardest part. The “after-action analysis” necessarily remains incomplete.