The legend of Lewis and Clark is today so deeply ingrained in our national memory, as the predecessors to the age of Davy Crockett and his wild frontier and to dying of dysentery on the Oregon Trail, that it's difficult to imagine a student of history not knowing about their historic journey. But our modern image of Lewis and Clark—exalted heroes of American exploration—is a fairly recent phenomenon. For nearly 150 years after their expedition, the nation almost forgot about Meriwether Lewis and William Clark completely.

"It really is an interesting rollercoaster, from the invisible to the iconic," explains James Ronda, the H. G. Barnard Chair in Western American History, emeritus at the University of Tulsa. "If you look all through the 19th century, they might be mentioned in a single line, even in to the 1920s and 30s, they end up getting wrapped up with the Louisiana Purchase, which is not what they were initially involved with."

Lewis and Clark were sent on their journey by President Thomas Jefferson, a man whose reputation spanned more than being the author of the Declaration of Independence: he was also a scholar of philosophy, language, science and innovation—interests that fueled his desire to learn more about the country in his charge. Jefferson had long dreamed of sending an expedition to the West—an idea that began, for him, around the end of the Revolutionary War. He attempted to send explorers West, across the Mississippi and Missouri Rivers, but none of these expeditions (one of which included George Roger Clark, William Clark's brother) came to fruition. Nonetheless, by the time he became president, Jefferson had amassed one of the largest libraries concerning the American West at his Monticello estate. Many of these books focused on North American geography, from The American Atlas: or, A Geographical Description of the Whole Continent of America by Thomas Jefferys to The Great or American Voyages by Theodor de Bry. All told, Jefferson had over 180 titles in his library on the subject of North American geography.

From his studies, one word came to define the West for Jefferson: symmetry. Jefferson viewed the West not as a wildly different place, but as an area dictated by the same geographical rules that reigned over the eastern United States—a kind of continental symmetry. His belief in such a symmetry contributed to the expedition's central assumption—the discovery of the Northwest Passage, a route that would connect the Missouri River with the Pacific Ocean. Because on the East Coast, the Appalachian Mountains are relatively close to the Atlantic, and the Mississippi connects with rivers like the Ohio, whose headwaters in turn mingle closely with the headwaters of the Potomac, providing a path to the Atlantic Ocean. Discovering such a passage to the Pacific was Lewis and Clark's primary objective; even as the two prepared for the journey by studying flora and fauna, Jefferson instructed Lewis to focus on finding "the most direct & practicable water communication across this continent for the purposes of commerce."

But the geography of the West turned out to be nothing like the geography of the East, and Lewis and Clark returned in September of 1806 without finding Jefferson's prized route. The mission—for these intents and purposes—was a failure. But Jefferson moved quickly to make sure that it wasn't viewed as such by the general public.

"What Jefferson did, very creatively, was to shift the meaning of the expedition away from the passage to the questions about science, about knowledge," Ronda explains. This was to be accomplished through Lewis' writings about the expedition, which were to be published in three volumes. But Lewis, for some reason, couldn't bring himself to write. At the time of Lewis' death, he hadn't managed to compose a single word of the volumes—and public interest in the expedition was quickly fading. Clark took the information gathered on the expedition and gave it to Nicholas Biddle, who eventually penned a report of the expedition in 1814. A mere 1,417 sets were published—essentially nothing, Ronda notes.

By the time Biddle's report was published, the country's attention had shifted to the War of 1812. In that war, they found a new hero: Andrew Jackson. Lewis and Clark sank further into obscurity, eventually replaced by John Charles Fremont, who explored much of the West (including what is now California and Oregon) throughout the 1840s and '50s, and ran for president in 1856. Materials that spoke to Lewis and Clark's accomplishments simply didn't exist, and the most useful resource of all—the expedition's original journals—were tucked away at the American Philosophical Society in Philadelphia. It's possible that, at that time, nobody even knew the journals existed. In American history books written for the country's centennial in 1876, Lewis and Clark have all but disappeared from the narrative.

Scholarly interest in the expedition begins to increase near the end of the 1890s, when Elliot Coues, a naturalist and army officer who knew about Lewis and Clark, used the expedition's only journals to create an annotated version of Biddle's 1814 report. At the beginning of the 20th century, with the expedition's centennial celebration in Portland, Oregon, public interest in Lewis and Clark begins to grow. "Now Lewis and Clark are beginning to reappear, but they're beginning to reappear as heroes," Ronda says.

In 1904 and 1905, Reuben G. Thwaites, one of the most distinguished historical writers of his time, decided to publish a full edition of the Lewis and Clark journals on the occasion of the centennial celebration of their trip. He thought that if more information was available about the expedition, public interest in the figures would increase. He was wrong. "It’s like dropping a stone in a pond and there are no ripples. Nothing happens," Ronda explains. Americans—historians and the public—weren't very interested in Lewis and Clark because they were still focused on understanding the Civil War.

Throughout the 1940s and into the early 1950s, distinguished historian and literary figure Bernard DeVoto tried to bring public attention to the history of the American West through a trilogy of historical books. The series culminated in 1952 with the publication of The Course of Empire, an account of Westward exploration which culminates in Lewis and Clark's expedition, and which won the National Book Award for Nonfiction. But, like Thwaites before him, DeVoto's timing was off: with much of the country still reeling from World War II, public interest in the series was limited.

It wasn't until the 1960s that the public and scholarly spheres connected to make Lewis and Clark the American icons they are today. In the academic world, the work of Donald Jackson changed the way the Lewis and Clark narrative was told. In the 1962 edition of the Lewis and Clark letters, Jackson wrote in his introduction that the Lewis and Clark expedition was more than the story of two men—it was the story of many people and cultures.

"What Donald did is to give us the bigger story," Ronda explains. "And now, there’s an audience."

Two events helped pique public interest in the Lewis and Clark story: the marking of the Western Trails by the federal government, which brought new attention to the country's history of Western exploration, and the founding of the Lewis and Clark Trail Heritage Foundation in 1969, whose stated mission is to honor and preserve the legacy of Lewis and Clark through education, research and preservation. "The 1960s were a tumultuous time. It was also a time of intense introspection about who we are as a people. One of those moments of introspection is wondering what is our history like?" Ronda explains.

In 1996, American historian Stephen Ambrose released Undaunted Courage, a nearly 600-page-long history of the expedition. The book was a New York Times #1 best-seller, and won both the Spur Award for Best Nonfiction Historical and the Ambassador Book Award for American Studies. Taking advantage of the wealth of new research uncovered by Lewis and Clark historians (especially Donald Jackson) since the 1960s, Ambrose's book was called a "a swiftly moving, full-dress treatment of the expedition" in its New York Times review (ironically, the same review touts Lewis and Clark as explorers who "for almost 200 years...have stood among the first ranks in the pantheon of American heroes"). The following year, Lewis and Clark's expedition was brought to life by the famed film maker Ken Burns in his four-hour PBS documentary Lewis & Clark: The Journey of the Corps of Discovery.

In terms of public interest in the Lewis and Clark expedition, Ronda feels that the 2006 bicentennial was the high-water mark—Americans celebrated all over the country with a three-year, 15-state pageant announced by President Bush. The Smithsonian's National Museum of Natural History ran a massive exhibit in 2003, featuring more than 400 artifacts from the expedition, the first time many had been in the same place since 1806. "Still, a lot of people still think about Lewis and Clark going out there all alone and there’s nobody else there. They don’t go into an empty place, they go into a place filled with native people, and the real story here is the encounter of peoples and cultures," he says. "You can understand the complexity of American life by using Lewis and Clark as a way to understand us as a complex people."

Editors' Note, October 2, 2014: This story has been edited to clarify and correct the explanation of Bernard deVoto's work. The Course of Empire came out in 1952, not the 1940s, and won a National Book Award for Nonfiction, not a Pulitzer Prize. He won the Pulitzer for an earlier work about the American West.