At some time in American history, every part of the United States except the easternmost region qualifed as "the West," the last frontier of settlement. As the idea of the West expanded in the public imagination, it became a land of cowboys, buffalo herds, a vast wilderness, and a society that operated on the other side of the law. The West was also the site of frontier warfare between settlers and American Indians. To a large extent, that picture was real, but the American West is far more than just a treasure trove of folklore. It is a vital part of the nation's growth and development, a rich depository of its history.

The age of western expansion refers mainly to the period following the Louisiana Purchase of 1803. Western expansion includes the lands acquired by treaty or warfare west of the Mississippi River...