Following the War of 1812, American entered a period of history known as the Era of Good Feeling. During this time, America articulated its first foreign policy through the Monroe Doctrine and a growing sense of nationalism gripped the population. Americans started believing that they were morally and culturally superior to their Native, Mexican, and British neighbors and that it was God’s will and the people’s duty to spread American virtues from "sea to shining sea."
In 1845, newspaper editor John O’ Sullivan first coined the term "Manifest Destiny," to describe the mindset of American expansion. The idea of Manifest Destiny was reflected in the 19th century by the mass migration of Americans from the eastern United States to the west through the great emigrant trails such as the Oregon, California, Santa Fe, and Mormon Trails. It was used to justify the corrupt treaties and wars that resulted in the removal of Native Americans from their ancestral land. It was also used to justify the Mexican-American War, resulting in the annexation of 1.2 million square miles of land in the American West.
Manifest Destiny also accelerated the onset of the Civil War as it touched off intense sectional dramas between southern states hoping to expand slavery into new territories and northern states hoping to prevent the spread of slavery.