What is the theme in Undaunted Courage by Stephen Ambrose?

Exploration and expansion are themes. The ideas of exploration and expansion form the core of Undaunted Courage—and the Lewis and Clark expedition. In 1790, the United States stretched only as far as the east bank of the Mississippi River. Though western lands were virtually unknown, they drew the interest of many Americans for several different reasons. At the opening of the nineteenth century, a pioneering spirit remained an integral part of the American character.