The vast majority of men and women who crossed the continent on the Oregon Trail in the mid-1800s did so for purely personal reasons. They were looking for a less crowded and more fertile place to farm, or they dreamed of getting rich in the goldfields, or they planned to make money selling trade goods. Yet, unknown to most of them, they were all a part of a grander scheme called Manifest Destiny.

Journalist John L. Sullivan coined the term in 1845, in an editorial that called for the United States to annex Texas. It was, Sullivan wrote, our country's "manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions."

Sullivan was one of a sizable number of writers, politicians, and ordinary citizens who called themselves "expansionists." The more extreme among them believed that...