In 1803, when Napoleon Bonaparte sold the 820,000-square-mile Louisiana Territory to President Thomas Jefferson, the French emperor effectively doubled the size of the twenty-seven-year-old American nation. With the stroke of a pen, Napoleon knowingly transformed the young United States into a force that the world would one day reckon with."The sale assures forever the power of the United States," Napoleon gloated. "I have given England a rival, who, sooner or later, will humble her pride."

When the French offered to sell the whole of Louisiana to the United States for a mere 15 million dollars, no one was more delighted - or surprised - than President Jefferson. After all, Jefferson's representatives had a more modest goal in mind when they first met with Napoleon in Paris; they wanted to acquire the city of New Orleans. Jefferson realized that securing this vital city at the mouth of the Mississippi River was essential for the young country's economic future. Any threat that Britain might capture New Orleans and strangle American trade along the Mississippi was unacceptable to Jefferson. Napoleon's decision to sell much more than New Orleans, however, was not sudden. Though the French once imagined that they could build their empire in the Americas, Napoleon readily admitted by 1803 that his ability to take on the British in this part of the world had faded. Instead, he now reasoned that his best opportunity to destabilize the British and their interests in the new country would be better secured by selling the whole Louisiana Territory to the Americans. When Napoleon made the offer, Jefferson grasped the opportunity. He quickly drew up a treaty for the transaction, a massive land deal that became known as the Louisiana Purchase.

The Louisiana Purchase more than doubled the size of the United States. It also gave Jefferson an even stronger reason to pursue a dream he had imagined for nearly two decades: a river-bound exploration and westward expedition to the Pacific Ocean. Even before the Louisiana Purchase, Jefferson had quietly planned such an expedition, with the help of modest funds secretly approved by Congress. Now, the President was free to increase the scope of the project and to talk openly about his goals for the expedition. Namely, to establish diplomatic relations with the Indians; to establish a foothold in the fur trade before the British could extend their own; to explore the length of the Missouri River (as well as its flora and fauna) westward from its mouth at the Mississippi; to search for the fabled Northwest Passage; and to record through books and maps all that was learned while crossing the unknown American continent. In essence, Jefferson dreamed of an expedition that would do nothing less than replace centuries of speculation with new knowledge. Was there a Northwest Passage to the Pacific by water? Were the Rocky Mountains one range or many? What would explorers encounter as they crossed the continent? Without doubt, the expedition Jefferson had in mind would provide answers to these questions and produce valuable knowledge about the unknown continent. And for Jefferson, the expedition had the potential to blunt the ever-present threats from England and Spain to capture America's Western frontier. "Science is my passion; politics is my duty," Jefferson admitted. With this expedition, Jefferson skillfully combined both.

Jefferson's interest in a transcontinental expedition dated back to the 1780s, when he was first elected to both the U.S. Congress and the American Philosophical Society. Through these positions, Jefferson learned that the British intended to investigate the part of the country that lay west of the Mississippi. Concerned that they would attempt to colonize this territory, Jefferson grew determined to explore the West. In 1783, he asked George Rogers Clark, the oldest brother of explorer William Clark, to lead an overland expedition on the North American continent. George Clark declined. In 1793, Jefferson convinced the American Philosophical Society and a handful of interested individuals - including George Washington - to fund another plan to explore the Pacific Northwest. This time, Jefferson directed botanist and seasoned explorer Andre Michaux "to find the shortest and most convenient route of communication between the United States and the Pacific Ocean." Jefferson asked Michaux to "take notice of the country you pass through," including its inhabitants, soil, minerals, animals, and mountains, "as they may be new to us and may also be useful." But this expedition had barely begun when it abruptly ended. Michaux, the serious explorer and botanist, also turned out to be a secret French agent.

Imagine Jefferson's increasing frustration, particularly when he learned that the British had finally pursued a plan he had long feared: an expedition designed to discover a northern route to the Pacific. In 1793, Scottish explorer Alexander Mackenzie led a team over the Canadian Rockies and crossed the Continental Divide. The story of this successful transcontinental adventure, published in 1801, confirmed Jefferson's worst fears: the British were knocking at America's back door.

When the members of the House of Representatives elected him President in 1800, Jefferson was finally in a position to meet this British challenge. At last, after years of dashed dreams and disappointments, he had the motivation, the knowledge, and now the authority to launch a serious American expedition. All he lacked, it seemed, was the opportunity to motivate others to feel the same urgency and determination. In 1803, thanks to Napoleon Bonaparte, Jefferson found that as well.

On July 4, 1803, newspapers published the announcement of the Louisiana Purchase and Jefferson's plans for an expedition to explore the newest corners of this young country. Immediately, all eyes turned to Jefferson's correspondence secretary, the frontiersman from Virginia who Jefferson had invited months earlier to lead the way - Meriwether Lewis. Overnight, it seemed, what began as a secret project became remarkably public, as well as a risky political and diplomatic event. By sponsoring an exploration of the Louisiana Territory, Jefferson set the stage for a breakthrough or, if it failed, a highly visible disaster. Jefferson acknowledged that the purchase "increased infinitely the interest we felt in the Expedition." But he also knew that the transaction with France was only so much paper. Unless the United States swiftly staked its claim to this land - and exerted physical and intellectual control of it - it would be difficult to expand across the continent and hold onto the new territory.

Jefferson had dreamed of an expedition far too long to imagine failure. He envisioned a group of ten to twelve explorers, led by Lewis. The object of your mission, Jefferson wrote to Lewis, "is to explore the Missouri river and such principal stream of it, as, by its course and communication with the waters of the Pacific Ocean, whether the Columbia, Oregon, Colorado or any other river may offer the most direct and practicable water communication across this continent for the purposes of commerce."

The story of the expedition begins in the East, in southeastern Pennsylvania. Before Lewis could proceed "on under a jentle brease up the Missouri," he had much to learn and much to prepare for. Lewis needed to acquire the technical and intellectual skills necessary not only to survive the expedition, but also to return with useful knowledge. Towards that goal, Jefferson insisted that Lewis consult the best scientific minds of the day, in Philadelphia and Lancaster. From the nation's most advanced scientists, Lewis acquired state-of-the-art knowledge in Indian language and culture, surveying, and the collection and description of natural specimens.

While Lewis spent weeks pursing the knowledge and practical advice necessary for the expedition, the actual planning for the trip proceeded slowly. With only a few months to go before the August departure date, Lewis was still the expedition's only "volunteer." By July, Lewis was relieved to know that William Clark - a trusted friend, fellow Virginian, and a seasoned but retired Army officer with significant experience as an Indian agent and frontier diplomat - had agreed to join Lewis and share in the leadership of the expedition. Though both men spent the next few months planning, preparing and recruiting the team of volunteers that would staff the expedition, only Lewis spent time in Pennsylvania. By the time Lewis started down the Ohio River from Pittsburgh on the last day of August 1803, cool weather was only a few weeks away, and the months of preliminary planning seemed rushed, sparse, and perhaps even inadequate. How could anyone know enough to prepare for a two-year expedition through vast, unknown, and potentially dangerous lands?

Lewis and Clark and their expedition volunteers joined forces for the first time on October 14th in Louisville, Kentucky, a stop along the Ohio River on their way to the Mississippi. By now, fellow explorer Patrick Gass had dubbed the Expedition the "Corps of Discovery." And what discoveries did this expedition claim? First and foremost, Lewis and Clark's twenty-eight month, 8,000-mile trek determined that the North American continent was about 1,200 miles broader than previously estimated. They learned that no Northwest Passage existed, and that the Rockies are not a single mountain range the size of the Appalachian Mountains but a complex series of much grander ranges, hundreds of miles wide.

Lewis and Clark also gathered a tremendous amount of valuable knowledge about the continent. They encountered many different Native American tribes and learned about their ways of life. They collected or identified 178 plants and 122 animals previously unknown east of the Mississippi. Some of this new knowledge was published in the years after the expedition. But today, some 200 years later, it continues to emerge.

Did the expedition itself change the American continent? It did not. As Lewis and Clark returned in the Autumn of 1806, they crossed paths with other Easterners heading West, those who would not wait to learn about what lay ahead or use the maps that would one day be printed. But the expedition did contribute, for better or worse, to a profound rethinking of the nation's shape, psyche, and destiny. No one, least of all Jefferson, could have imagined how rapidly the opening of the American West would reshape the nation. Jefferson himself estimated that at least one hundred generations would pass before the vast expanse of the West was populated. In reality, it took only five.

In the 18th century, the United States found its political origins in Pennsylvania. Now, at the turn of the nineteenth century, the nation was beginning again, looking not to Britain and the East, but to the continent and the West. In the very real terms of ideas, supplies, and transportation, this newer nation - the one envisioned by the leaders of the Lewis and Clark Expedition - also began in Pennsylvania.