Between 1750 and 1850, pioneers pushed America's borders from a few colonies on the Atlantic all the way to California and Oregon on the Pacific. The new nation ultimately encompassed more than 3 million square miles. This vast territory, with its rich natural resources, made America the world power it is today.

During the 1700s the American colonies extended only about two hundred miles inland from the Atlantic coast. For the previous 150 years, the early colonial settlers had ventured no farther west than the Appalachian Mountains.These mountains were a natural barrier that stopped colonial expansion. Only fur traders and trappers traveled into the wilderness of the Ohio Valley. Native American tribes like the Ottawa, who lived west of the mountains also violently opposed any settlement in their territory. Indian attacks pushed the early pioneers back to the eastern side of the mountains.

But the lure of free...