At the dawn of the nineteenth century, the size of the United States d oubles 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 Libe rty begins to take shape. American historian Stephen Ambrose joins Pro fessors Maier and Miller in examining the consequences of the Louisian a Purchase -- for the North, the South, and the history of the country .