In 1845, New York journalist John O'Sullivan coined the phrase "Manifest Destiny," an expression that would remain shorthand during the late nineteenth century for the belief that Americans had an obligation to settle the western territories. Indeed, the phrase "Manifest Destiny" implied that America's expansion was predetermined, undeniable, and--most importantly--inspired by God.

The ideas that precipitated talk about and belief in Manifest Destiny, however, were not necessarily the most important cause for America's population expansion in the west. Rather, throughout the late 1840s, Manifest Destiny was a rallying cry of those in government who wanted the entirety of ...