Manifest Destiny: The Idea of Westward Expansion

Manifest Destiny was the belief, widespread in the 1840s, that the United States was destined to expand across North America to the Pacific. The journalist John L. O'Sullivan popularized the phrase in 1845. Supporters used it to justify the annexation of Texas (1845), the Oregon settlement with Britain (1846) and the Mexican-American War (1846–48), after which Mexico ceded a vast territory.

Critics at the time, including Abraham Lincoln and Henry David Thoreau, opposed the war. For Native American nations, expansion meant the loss of homelands, forced removals and broken treaties, and the question of whether new western land would allow slavery deepened the divisions that led to the Civil War.