Was the American expansion across North America an "inevitable" development? How was the idea of Manifest Destiny used to justify expansionism?

Manifest destiny was the 19th century U. S. belief that the country (and more specifically, the white Anglo-Saxon race within it) was destined to expand across the continent. Democrats used the term in the 1840s to justify the war with Mexico. The concept was largely denounced by Whigs and fell into disuse after the mid-19th century. Advocates of manifest destiny believed that expansion was not only wise, but that it was readily apparent (manifest) and could not be prevented (destiny).

The concept of U. S. expansionism is in fact much older. It is rooted in European nations' early colonization of the Americas, the establishment of the United States by white Anglo-Saxons from England, and the continued wars against and forced removal of the American Indians indigenous to the lands. In 1845, John L. O'Sullivan, a New York newspaper editor, introduced the concept of "manifest destiny" in the July/August issue of the United States Magazine and Democratic Review, in an article titled, "Annexation." The term described the very popular idea of the special role of the United States in overtaking the continent-the divine right and duty of white Americans to seize and settle the continent's western territory, thus spreading Protestant, democratic values.