It is said that all stories have two sides. In the best stories, the two sides are inseparable. Pull them apart, and it makes the whole thing meaningless. "[The expedition] has that mixed quality of great news for one people and bad news for another gr oup of people," says Patricia Limerick, who chairs the board of the Center of the American West at the University of Colorado in Boulder. "It is not the greatest news," she says, "to have a party of agents of empire come through."

In the long run, Limerick is right, of course, but the Lewis and Clark expedition was really a series of short runs placed end to end until it stretched all the way to the Pacific. At a time when Americans have every reason to fear what's waiting for them down the trail, from enemy armies to our capacity for misunderstanding and miscalculation, it's important to remember that what's to come is first a matter of what one does today, here, on this spot. All those footsteps will add up.

Even with the addition of Sacagawea, who had lived in the regions the expedition had yet to cross, the corps was not always sure where it was going, but its members were keenly aware of where they stood at every important moment along the way. Lewis and C lark looked around, not just ahead — at prairie dogs in their burrows, at herds of buffalo massing in grassy valleys, at lights in the sky and seedlings in the soil.

And they took the time to write down in their journals everything they saw. If not for the piecemeal epic the captains scratched out while crouching on hillsides and squatting on riverbanks, we might not remember Lewis and Clark at all. "There are a lot o f very terse diarists in the world who say, 'Proceeded up river and camped,'" says Limerick. "We're very lucky to be their heirs because of their fluidity of words."

Their words didn't grab the nation's attention immediately. The first edition of the journals didn't appear until eight years after the expedition ended, in 1814. Hundreds of books later, it's hard to imagine the absence of Lewis and Clark from the pagean t of popular American history. Without them, there would still be stirring tales of exploration but none that turn on the exquisite irony of an adolescent Indian girl giving crucial advice to two male Army officers. There would still be images of frontier adversity but none so stunning as that of Lewis expecting to see a path to the Pacific but discovering endless ranks of mountains instead. There would still be historical markers on western highways but none that lead thousands of miles to the sea and al low the pilgrim at every stop to cross-reference the vista spread out before him with the written impressions of those who blazed the trail.

"It's the emblematic American journey," says Ronda. "In U.S. history there is always a tension between home and the road. We talk a good talk about the joys of home, but the truth is we are obsessed with the road."

Like every road, this one goes both ways. The country that Lewis and Clark returned through was not the same country they had just crossed. Its rivers had been named, its plants and animals sketched and classified, its native people apprised of their new status as subjects of a distant government whose claim to the place consisted of a document — the Louisiana Purchase — that none of its actual inhabitants had signed.

The tribes reacted differently to their changed positions in the new order. The Nez Perce, who had considered killing Lewis and Clark when they first spotted them limping out of the mountains in what is now Idaho, welcomed them like lost brothers on their return trip, offering idealistic pledges of permanent friendship with the U.S., whose citizens would later repay the gesture by forcing the tribe from its hunting and grazing grounds and corralling its weakened remnants on reservations. The Blackfeet had a touchier response, perhaps because their unrivaled dominance on the northern plains was threatened by the Americans' plans to begin trading with the neighboring tribes. One morning, while camping in what is now Montana, Lewis awoke to a struggle betwee n an underling and an Indian who was trying to steal a rifle. Moments later, one Blackfoot brave lay fatally stabbed, and another was bleeding from the gut, cut down by a bullet from Lewis' gun.

Aside from the captains' early floggings of disobedient underlings, this was the party's only violent act. More remarkable, perhaps, is how much violence the explorers avoided, despite their varied ethnic and racial backgrounds and the ceaseless frustrati ons of the trip. It's an inspiring thought — the melting pot on the march — but like most simple images of the famous journey, it doesn't tell the full story, or even half of it. For every uplifting aspect of the tale, there's a difficult, melan choly sidelight, which may well be the secret of its abiding power.

After 8,000 miles and 28 months of travel from their start near St. Louis, the corps returned to a hero's welcome as joyful as it was short-lived. Jefferson, according to historians, soon grew disappointed in the enterprise. It had failed to substantiate his western dreams of a well-watered garden convenient to the Pacific where generations of self-sufficient farmers would live in democratic bliss, free from old, corrosive political controversies such as slavery. As for peace with the Indians, and among t he Indians, well, those medals certainly were handsome. And then there was Lewis, of course, the chronic depressive who may have reached his spiritual high point somewhere back along the wild Missouri. In 1809, while on his way to Washington to defend his expense report to a bureaucrat in the War Department, he lay down in a Tennessee inn and shot himself.

Some people are better at leaving than at returning.

But who knew? Not Lewis, not Jefferson, not the Indians. In July 2002 we don't know either. Particularly since the events of last September, we sense that there's something enormous and strange ahead of us — in the darkness, over the mountains, throu gh the trees — but we have no idea what it is or how far off. To find it, face it and live to write the story, we'll have to be resourceful, lucky, patient, flexible and observant, much as Lewis and Clark were. We'll have to row into the current of o ur ignorance, one stroke at a time.

With reporting by Deirdre van Dyk/New York