manifest destiny, belief held by many Americans in the 1840s that the United States was destined to expand across the continent, by force, as used against Native Americans, if necessary. The controversy over slavery further fueled expansionism, as the North and South each wanted the nation to admit new states that supported its section's economic, political, and slave policies. By the end of the 19th cent., this belief was used to support expansion in the Caribbean and the Pacific.