With the Louisiana Purchase in 1803, the territory of the United States doubled overnight. Months before the $15 million deal was finalized, though, President Thomas Jefferson won approval from Congress to send a team of intrepid explorers to find a passable water route west to the Pacific Ocean.

Jefferson tapped his personal secretary, Meriwether Lewis, to lead the “Corps of Discovery,” and once Lewis grasped the full scope and challenges of the expedition, he called on his Army friend and fellow Virginian, William Clark, to be his equal in command.

“If therefore there is anything… in this enterprise, which would induce you to participate with me in it’s fatiegues, it’s dangers and it’s honors,” Lewis wrote to Clark, “believe me there is no man on earth with whom I should feel equal pleasure in sharing them as with yourself.”

Below is a timeline of Lewis and Clark's extraordinary expedition.

May 14, 1804

The Corps of Discovery embarks from Camp Dubois outside of St. Louis, Missouri, in a 55-foot keelboat to begin the westward journey up the Missouri River. Among the 41-man crew of volunteers, soldiers and one African American slave, is Patrick Gass, a carpenter from Pennsylvania. Gass writes in his journal about the expected dangers ahead, including “warlike nations of savages of gigantic stature” and impassable mountain ranges.

“The determined and resolute character, however, of the corps, and the confidence which pervaded all ranks dispelled every emotion of fear, and anxiety for the present,” writes Gass, “[and] seemed to insure to us ample support in our future toils, suffering and dangers.”

August 20, 1804

Sergeant Charles Floyd, the youngest man on the expedition, dies of a suspected ruptured appendix near modern-day Sioux City, Iowa. Incredibly, Floyd’s is the only death during the entire two-year expedition.

September 25, 1804

Of all Lewis and Clark’s encounters with Native American tribes, the meeting with the Teton Sioux (Lakota) near modern-day Pierre, South Dakota, is among the most tense. Jefferson had charged the Corps with Indian diplomacy, which consisted mainly of announcing the Louisiana Purchase and presenting tribal chiefs with peace medals and American flags.

But communication breakdowns are common during the expedition, given that Lewis and Clark often rely on three-way translation (native language to French to English and back) or sign language to converse with chiefs who often have their own political agendas.

On this day, the Teton Sioux mistake the explorers for merchants and don’t like the idea of the Americans selling weapons to rival tribes up the Missouri River. A young Teton Sioux chief, trying to insert himself into the confrontation, feigns drunkenness and stumbles into Clark, who rashly draws his sword. In an instant, Clark’s soldiers raise their rifles and the Teton braves draw their bows and arrows.

After exchanging angry threats and boasts through nervous interpreters—at one point Clark claims that he has "more ‘medicine’ on board [his] boat than would kill twenty such nations in one day"—the elder Chief Black Buffalo breaks the tension and calls for peace. After three restless days at the Teton Sioux village, the upriver journey is allowed to proceed.

November 11, 1804

With winter fast approaching, the Corps construct Fort Mandan in North Dakota among the hospitable Mandan and Hitatsa Indians. On November 11, Clark makes a hasty scribble in his journal about the arrival of "two Squars of the Rock Mountain, purchased from the Indians by...a frenchmen." One of those nameless women is the famous Sacagawea.

At first, Sacagawea is an afterthought. She is the 17-year-old, pregnant wife of Toussaint Charbonneau, a French Canadian trader hired by Lewis and Clark as a Hidatsa interpreter. But she soon proves to be an invaluable member of the expedition.

“Sacagawea helped [Lewis and Clark] in a number of ways,” says Jay Buckley, a history professor at Brigham Young University and author of several books about Western exploration. “Both in letting native tribes know that they came in peace, as well as helping the men with their diet, finding edible plants to improve their health.”

Two days after the Corps depart Fort Mandan in the spring of 1805, Lewis writes in his journal that “[Sacagawea] busied herself in serching for… wild artichokes… by penetrating the earth with a sharp stick… her labour soon proved successful, and she procurrd a good quantity of these roots.”

June 2, 1805

Lewis and Clark rely largely on navigation tips from Indians and white traders to chart the fastest and safest route westward toward the Pacific. But they are fully unprepared for a major fork in the Missouri River in north-central Montana. Only one fork is the true Missouri, and they will know it by a series of majestic waterfalls upstream mentioned by the Mandan-Hidatsa.

Lewis and Clark call for a vote. Thirty-one people vote for the right fork and only two vote for the left—those two were Lewis and Clark. Not willing to defy their men, Lewis and Clark send exploring parties up each fork and have them report back. A second vote is taken with exactly the same result.

“But the men, to their credit, say, ‘We’re going to follow you,’” says Buckley.

June 13, 1805

Anxious to prove he’s right, Lewis scouts ahead of the rest of the Corps and is overjoyed (at first) to find the Great Falls, describing them as a “truly magnifficent and sublimely grand object, which has from the commencement of time been concealed from the view of civilized man."

But it soon becomes clear that the portage (carrying canoes over land) around the Great Falls is going to be far more difficult and will require more than the one day he planned. To help with challenge, the men fashion crude wagons from felled trees and drag the canoes and equipment across miles of unforgiving, cactus-strewn terrain.

“It takes them almost a month and a half to take all of their gear 18 miles,” says Buckley. “It’s probably one of the slowest parts of the whole trip.”

August 8, 1805

Before she was kidnapped by the Hidatsa at age 12, Sacagawea lived among the Shoshone people along the border of modern-day Montana and Idaho. By August, 1805, Lewis and Clark believe the fate of the expedition hangs on finding the Shoshone and buying horses from them. It’s the only way the Corps can hope to cross the Rocky Mountains before winter.

While Sacagawea doesn’t “guide” the expedition, her childhood memories provide valuable clues that they are on the right path. On August 8, Lewis writes in his journal:

“The Indian woman recognized the point of a high plain to our right which she informed us was not very distant from the summer retreat of her nation on a river beyond the mountains. . . . this hill she says her nation calls the beaver's head from a conceived resemblance. . . . she assures us that we shall either find her people on this river on the river immediately west of it's source. . . . as it is now all important with us to meet with those people as soon as possible.”

August 17, 1805

After Lewis and Clark finally make contact with the Shoshone, Sacagawea is joyfully reunited with her brother Cameahwait, who is now the Shoshone chief.

September 11, 1805

Even with horses and a Shoshone guide named Old Toby, the crossing of the Bitterroot Mountains in Idaho proves to be the most grueling and life-threatening section of the entire journey.

It was only mid-September, but the snow on the western flank of the Bitterroots is already deep and Old Toby gets lost. Horses slip and tumble down the mountain. The men, who have grown accustomed to eating five to seven pounds of meat daily in the game-rich plains, begin to starve. They become so desperate they start eating the colts.

Eleven days later, they stumble out of the forest snow-blind and weak with hunger, and are taken in by a village of Nez Perce Indians. Buckley says that the Nez Perce debate killing the half-dead intruders, who are accompanied by a Shoshone woman, their bitter enemy. But a Nez Perce woman named Watkueis, who lives among white men as a captive, convinces them to spare the strangers and befriend them.

The Nez Perce hospitality has one drawback. Lewis and Clark’s men make themselves sick from overindulging on piles of dried fish and boiled roots.

Clark writes in his journal, “I find myself verry unwell all the evening from eateing the fish & roots too freely.” A week later, he adds, “Capt Lewis & myself eate a Supper of roots boiled, which Swelled us in Such a manner that we were Scercely able to breath for Several hours.”

November 7, 1805

After paddling dugout canoes down the treacherous Columbia River for weeks, Clark believes the men have finally reached the Pacific.

“Great joy in camp we are in View of the Ocian,” writes Clark with his trademark creative spelling. “This great Pacific Octean which we been So long anxious to See. and the roreing or noise made by the waves brakeing on the rockey Shores (as I Suppose) may be heard distictly.”

Alas, to Clark’s dismay, they have only arrived at the edge of Gray’s Bay, a storm-tossed brackish estuary 20 miles inland from the Pacific. Powerful waves and strong winds swamp and paralyze the canoes.

In a typical journal entry marked November 14, Clark writes, “rained all the last night without intermition, and this morning. wind blows verry hard but our Situation is Such that we Cannot tell from what point it comes—one of our Canoes is much broken by the waves dashing it against the rocks.”

The Corps finally crosses the estuary with the help of local Clatsop Indians and their large, ocean-going canoes.

November 24

After finally reaching the Pacific Coast, it‘s time to hunker down in Winter quarters. Lewis and Clark put the decision to a vote as to where to build Fort Clatsop, their home for the next five months. A tally of the votes is recorded in Clark’s journal, including historic votes from York, the African American slave, and Sacagawea, an Indian woman.

“Janey [one of Sacagawea’s nicknames] is in favour of a place where there is plenty of Potas,” wrote Clark, referring to Wapato, a type of native root vegetable.

December, 1805 - March, 1806

Conditions at Fort Clatsop are “horribly miserable,” says Buckley. “From December to March, it rained all but 12 days. They were stuck in cramped, smoky quarters subsisting on lean elk meat. They were ready to start the return journey as soon as they thought possible, and they actually left too soon.”

May, 1806

Returning to the Nez Perce, Lewis and Clark go against the natives’ advice and try to cross the thickly forested Bitterroots before the snow fully melts. “It was their only retreat during the whole expedition,” says Buckley.

June 8, 1806

The month spent with the Nez Perce waiting for the snow to melt is one of the most enjoyable and leisurely of the entire two-year journey. By day, Lewis and Clark’s men and the Indians compete in foot races and boyhood games like “prison base,” a type of tag. And by night, they stay up late dancing and playing the fiddle around the fire.

"Last evening the indians entertained us with seting the fir trees on fire,” writes Lewis, describing one of their last nights among the Nez Perce. “This exhibition reminded me of a display of fireworks. The natives told us that their object in seting those trees on fire was to bring fair weather for our journey."

July 3, 1806

After easily crossing the Bitterroots with the help of Nez Perce guides, the Corps split into four different groups for the next leg of the journey. Clark leads a group to explore the Yellowstone River. Lewis takes another up the Marias River, which includes the northernmost edge of the Louisiana Territory. Without any way to communicate with each other, they plan to reunite at Fort Mandan.

July 25, 1806

Clark etches his name and the date into a sandstone outcropping near modern-day Billings, Montana that he names Pompy’s Tower after Sacagawea’s son. It remains rare physical evidence of Lewis and Clark’s expedition that survives today.

July 26, 1806

Lewis’s group is met by a small band of Blackfeet warriors in Montana. After camping together overnight, Lewis catches the Blackfeet trying to steal their guns and horses, and kills a young brave.

“That was the only native death on the whole expedition,” says Buckley. “And Lewis was so concerned that the Blackfeet would come after him, he and his men jump on their horses and ride for almost 24 hours straight to get down to the Missouri River and meet up with the rest of the party.”

September 23, 1806

A month after Lewis and Clark reunite at the confluence of the Missouri and Yellowstone, and weeks after saying goodbye to Sacagawea at Fort Mandan, the Corps of Discovery arrive back in St. Louis, where the exhausted explorers are greeted as heroes.

“Even though there were all these difficulties with mountains and rivers and climates and natives, they all live—they all come back,” says Buckley. “And the Lewis and Clark expedition becomes America's odyssey.”