Lewis and Clark are important to American History because their efforts defined the expanded United States.
With the Louisiana Purchase from France, President Thomas Jefferson made a significant investment in the nation's future. The nation nearly doubled in size. However, Jefferson was not clear as to what he purchased. Lewis and Clark were critical in detailing the President's acquisition.
Lewis and Clark are important to American History because of their discoveries on their expedition through the territory that constituted the Louisiana Purchase. Jefferson himself highlighted the goal and importance of their mission:
The object of your mission is to explore the Missouri River, & such principle 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 & practicable water communication across this continent for the purpose of commerce. (National Park Service, "Lewis and Clark")
In traveling thousands of miles in two and one-half years, a relatively small period of time, Lewis and Clark failed to accomplish what the President asked of them. Their expedition demonstrated there is no viable water path across the continent to the Western portion of the country.
They forever destroyed the dream of a Northwest Passage (a water route across the continent), but proved the success of overland travel to the Pacific.
Another reason why Lewis and Clark are important to American History is because of the partnership they forged with Sacagawea. Her role in the expedition was vitally important. She functioned as guide, diplomat, and translator. The relationships that both men formed with indigenous people were enhanced because of Sacagawea's presence. Their association with her represents another reason why Lewis and Clark were important to American History.