Overview: At the dawn of the 19th century, the size of the United States doubles with the Louisiana Purchase. The Appalachians are no longer the barrier to American migration west; the Mississippi River becomes the country's central artery; and Jefferson's vision of an Empire of Liberty begins to take shape. Historian Stephen Ambrose joins Maier and Miller in examining the consequences of the Louisiana Purchase – for the North, the South, and the history of the country.