At the turn of the eighteenth century the territory east of the Mississippi River seemed like enough land for the growing U.S. population for generations to come. Early explorers had indicated that much of the land west of the Mississippi was either too arid or too mountainous to serve a nation of farmers. In fact, many maps depicted the area west of the Mississippi as the Great American Desert. But wars and the discovery of gold in the West soon led to a hunger for expansion. Settlers who had ventured into Texas (before the territory became a state) found themselves at odds with the Mexican governors of the territory. Eventually Americans joined the dispute, fighting a war with Mexico that earned the United States a vast territory, including present-day Texas, New Mexico, Arizona, Nevada, and California. (See...